Remove noise_levels and templates from quality metric deps

[chrishalcrow]

At the moment, the quality_metrics extension requires noise_levels and templates to be computed before we can compute quality metrics. However,

1. Only sd_ratio depends on templates.
2. Only snr depends on noise_levels.
3. Other metrics depend on other extensions.

So these requirements are a bit over the top. This PR moves the extension existence check to the compute_metric functions, and adds a nice function _has_required_extensions which deals with the error message. Does not work for the amplitude metrics, since they can use spike_amplitudes or amplitude_scalings.

Now, the quality metrics extension has no dependencies, and will compute some spike-train related metrics if you give it an extension-less analyzer.

We had discussed introducing a UnitMetric class or something. I think this is simpler for now!