kimber grace and rain threshold implementation differences

[mikofski]

Describe the bug

1. the threshold in pvlib is strict...

   pvlib-python/pvlib/soiling.py

   Line 208 in 0f7a205

   rain_events = accumulated_rainfall > cleaning_threshold

   ...whereas the paper Kimber, 2006 implies that rainfall inclusive of the threshold is sufficient to clean:
   

2. the grace period has an off-by-one error, including the rain day and therefore effectively reducing grace days to gracedays-1

To Reproduce

1. set rainfall to exactly the value of rainfall in the accumulation period and it will not clean
2. set grace days to 1, and effectively here are no grace days!

Expected behavior

1. rainfall equal to should clean
2. grace day of 1 should be clean for 1 day after rainfall event

Versions:

• pvlib.__version__: 0.15.2

Additional context
Thank you @wholmgren for pointing out these inconsistencies!

May also be related to

• pvlib.soiling.hsu does not require rain_accum_period to be a Timedelta #1711
• Kimber Dynamic Soiling model #669