Add more backwards compatibly for MockWaveformExtractor

[chrishalcrow]

Added several methods to MockWaveformExtractor and changed arguments in compute_blah_extensions function.

Will keep in draft until tests are added.

Added:

• format
• get_available_extension_names
• precompute_templates
• sample_spikes
• run_extract_waveforms
• select_units
• delete_extension
• get_random_spikes (new - without a way of computing some random spikes, you can't compute templates and other things)

to MockWaveformExtractor

I changed the FunctionFactory for the compute_blah_extensions (e.g. compute_principal_components, compute_templates) for backwards compatibility. Previously these took the form compute_principal_components(waveform_extractor=we) as well ascompute_principal_components(we).

Not added some methods. These involve:

• Extension classes (extensions are now objects of a new extension class) so it's hard to see how these will not break (extensions, get_extension_class, is_extension, load_extension, register_extension)
• Loading stuff after loading your initial MockWaveformExtractor (load, load_from_folder, load_from zarr) -Segments since SortingAnalyzer seems to deal with segments a bit differently (get_template_segment and get_waveform_segment).