[DSG] - Spike Train Sampling rates

[TomDonoghue]

Spike trains have a sampling rate, but right now when we use them, this is little impicit (defaults to 1kHz, but doesn't document this, and has no options for changing.) This should be documented, and made more generalizable

Functions to check for this:

• convert_train_to_times
• create_spike_train
• maybe others (to check)

Also: computing spike trains depends on the unit of the data (ms vs. second), and the start time of the task. For examples, it blows up if times are encoded in UNIX time (as it tries to create a spike_train from time 0).

Note: relates to #5

[TomDonoghue]

This is now done in the linked PRs.