Fix bug in get_repolarization_slope with wrong index type

[zm711]

In the low-level implementation of get_repolarization_slope line 471/3:

       trough_idx = get_trough_and_peak_idx(template_single)

This will return a tuple instead of an int, which will cause the slicing to break later in the function.

Since the high level call for template metrics supplies trough_idx and peak_idx this bug would only appear if someone tried to call the low level function by itself.

Also edited some docstrings.

[alejoe91]

Thanks Zach!