every quality metric should be an independent extension

[rat-h]

After switching to a new interface for postprocessing, aka SortingAnalyzer, it seems awkward and artificial to keep quality_metrics as an extension with a bunch of metrics inside. Currently extensions can be seen as a tree:

waveforms
|   \- Parameters for waveforms
spike_amplitudes
|   \- Parameters for spike amplitudes
quality_metrics
     \- list of metrics
     \- metric parameters
     |   \- parameters for isi_violation
     |   \- parameters for sd_ratio
    ....

It seems more natural to have a flat tree of extensions like that

waveforms
|   \- Parameters for waveforms
spike_amplitudes
|   \- Parameters for spike amplitudes
isi_violation
|   \- parameters for isi_violation
sd_ratio
|   \- parameters for sd_ratio
....

Moreover, we can preserve the current quality_metrics interface as a wrapper and aggregator of different metrics.

Does it make any sense?
rth

[zm711]

@chrishalcrow this was your proposal no? Trying to find the best way to give more flexibility in doing the qc metrics.