Fix RoboLowess default temporal_resolution handling (#137)

[oindabi]

Fixes a TypeError in RoboLowess.fit() when using default settings and
temporal_resolution=None. The code now falls back to a valid positive
time resolution when win_size is auto-computed.

[AdamGoldstein-USRA]

This looks good to me. Asked Josh to review to ensure it takes care of his issue.

[joshuarwood]

This fix avoids the crash, but it seems like setting a default temporal_resolution=1.0 would be easier to maintain than the nested if/else statements.

Is there a specific reason to define temporal resolution from the binning?

I don't see any changes to unit tests. We should add or update one of the tests to use the default settings so that issues like this are caught in the future.