Add usage examples for reactive operators

[banchan86]

Partially addresses bonsai-rx/roadmap#15

Progress

37/100 Bonsai.Reactive operators (I am not sure all of them need examples)
The examples have been updated with the VisualizerWindow from Bonsai 2.9.0
Operators covered (click on operator link for online preview - some might not be up yet till I update the demo branch):

Operator	Category	Basic Example	Application Examples	Alternatives
Accumulate	Quantitative	✓		Sum
Average	Quantitative	✓	Signal Smoothing
Count	Quantitative	✓		ElementIndex
Min	Quantitative	✓
Max	Quantitative	✓
Sum	Quantitative	✓		Accumulate
CombineLatest	Combine	✓	Video Synchronization	WithLatestFrom
Concat	Combine	✓	Stimulus Composition	Merge
Merge	Combine	✓	Input Aggregation	Zip CombineLatest WithLatestFrom Concat
WithLatestFrom	Combine	✓	Video Synchronization Background Subtraction	CombineLatest
Zip	Combine	✓	Combine Processing Results Combine Related Events Sample Preloaded Sequences	WithLatestFrom CombineLatest
Gate	Dynamic Filter	✓		GateInterval WindowTrigger BufferTrigger
Sample	Dynamic Filter	✓		SampleInterval Zip
SkipUntil	Dynamic Filter	✓	Start Video Recording	SubscribeWhen
SubscribeWhen	Dynamic Filter	✓		SkipUntil
TakeUntil	Dynamic Filter	✓	Stop Video Recording
Condition	Filter	✓
Distinct	Filter	✓		DistinctUntilChanged
DistinctUntilChanged	Filter	✓	Signal State Transitions
First	Filter	✓		Take FirstOrDefault
FirstOrDefault	Filter	✓		Take
Last	Filter	✓		TakeLast LastOrDefault
LastOrDefault	Filter	✓		TakeLast
Slice	Filter	✓	Deinterleave Video Frames	GateInterval
Skip	Filter	✓	Discard Startup Elements	Slice
SkipLast	Filter	✓		Delay
Take	Filter	✓		First
TakeLast	Filter	✓		Last
CreateTimestamped	Temporal	*
Delay	Temporal	✓		DelaySubscription
DelaySubscription	Temporal	✓		Delay
GateInterval	Temporal	✓		SampleInterval Slice Gate
SampleInterval	Temporal	✓		GateInterval Slice Sample
Throttle	Temporal	*
Timer	Temporal	✓
TimeInterval	Temporal	✓
Timestamp	Temporal	✓	Record Data Timestamps

General Issues

• Haven't decided whether to add higher-order examples.
• In many cases, I think the description for the basic example closely mirrors the editor description for the operator, so I might just take those out and leave only descriptions for the application examples. I will probably leave it for one of the last edits so it can be reversed if necessary.
• The section that follows that follows with the class declaration code snippet, inheritance and inherited members does not have a section heading. That makes it seem to be a part of the examples or application examples. I think adding a section heading is needed, but I will also probably add it as an edit near the end.
• Some reactive operators already have a section talking about alternatives and even some example workflows, was not sure if I should merge them with the new sections I am adding.

Operator-specific Issues (*)

• For Throttle, it was hard to build a basic example using just Bonsai.Core reactive operators, since it relies on an input with variable rate, so I used KeyDown as an input.
• CreateTimestamped was renamed from CombineTimestamp but the accompanying doc files were not updated yet, these files were renamed. In addition, the existing example (to carry forward the timestamp) was adopted as a basic example. While it is more like an application example, I could not figure out how to generate a basic example without copying much of that workflow. In addition, a true basic example for this operator would rely on timestamps coming from a different source.

[banchan86]

I updated this PR with the feedback from DCM and added another batch of reactive operators (operators that "combine" sequences). As such i changed the title of this PR and also added a chart in the top level comment to document which operators were included.

Other changes that were made:

• Switched basic types in basic examples to those from Bonsai.Core (Range/Int/Timer)
• Separated "basic examples" from "practical application examples"
  • One point we discussed was whether to even include practical/complex examples or leave them out. As I was covering the "combine" operators however, I discovered that there was already some mention of practical applications for the operators in the overwrite articles. Distinguishing between those operators was also hard with just basic examples. Thus, I opted to include practical examples/applications for them by simply expanding on those existing notes with workflow containers.
• Improved visualization of operators that only emit a final value by repeating the block
  • I noticed that the previous solution of using a delay node and a note to use ObjectTextVisualizer was very clunky and unintuitive.