extend filter configuration for fast out-of-core processing

[eackermann]

Currently filter epochs are computed internally, and the API is not properly exposed to allow a user to specify in-core or out-of-core, etc.

In addition, it may be desirable to pass in desired epochs (with index=True) directly to the filter object, so that they won't have to be recomputed every time we want to filter. This approach can also make it simpler to customize the epoch calculation to suit the application needs.

[eackermann]

As an example, we may want something along the lines of

# compute epochs on giant timestamps object that's too large to process in-core:
epochs = jag.utils.get_contiguous_segments(timestamps, assume_sorted=True, step=1, in-core=False, index=True) 
# filter into some band:
filtered = jag.filtering.filtfilt(data, **params1, epochs=epochs)
# now filter into another band, without having to recompute the epochs
filtered2 = jag.filtering.filtfilt(data, **params2, epochs=epochs)

[jchutrue]

Epochs can be passed into filtfilt_within_epochs_mmap--see first commit on the dev branch