Alignment of feature windows when equal stride

[jvdd]

There should at least be an option to conveniently align the feature windows when there is an equal stride (or multiples of the minimal stride).

This issue persists when the windows have different length (e.g., "3 min" and "5min") and have an equal stride (e.g., "2min"). The user would want to have the features aligned on the same index (e.g., after 5 min), and not misaligned (as it is right now) after 3min and 5 min.

I guess we could introduce a keyword argument in the FeatureCollection.calculate for this.

Disclaimer: issue was originally highlighted by @emield12

[jvdd]

PR #62 tackles this (partially) by setting the window_idx="begin" default instead of "end"!