pvlib.soiling.hsu does not require rain_accum_period to be a Timedelta

[williamhobbs]

Describe the bug
If a user accidentally passes rain_accum_period in pvlib.soiling.hsu as an integer, the function runs without error. Is that desired? I'm concerned that someone could test with hour interval data, in which case rain_accum_period = 1 and rain_accum_period = pd.Timedelta('1h') will give the same results. But, if they later switch to using, e.g., 15-min interval data, rain_accum_period = 1 may give unintended results.

Expected behavior
Either an error for using integers, or clarification in the function's documentation that integers will work but they specify number of intervals instead of number of hours.

Tagging @andrewtrendline