Add some wavelength-dependent effects

[dkirkby]

This is not ready to merge yet, but I wanted to share the changes I am making so you can follow along if you are interested.

The plan is to include a filter-dependent distribution of initial electron positions to study the wavelength dependence of simulation results. Are there other wavelength effects we should look at?

The initial changes are just housekeeping:

• Add an appropriate file extension to output files (.hdf5 or .dat).
• Add git rules to ignore binary output files.

[dkirkby]

I noticed that a lot of the "changes" in my commits are due to my editor (atom) removing invisible white space from source code, which is probably a good idea but lowers the SNR for browsing the commits. Sorry about that!

[dkirkby]

@craiglagegit The Pts file is empty when I run examples 1 and 2 now, which causes Poisson_Plots.py to report an error. I will look into more, but wondered if you had any ideas?