Fix pandas 1.3.0 error and deprecations

[kandersolar]

• Code changes are covered by tests
• New functions added to __init__.py
• API.rst is up to date, along with other sphinx docs pages
• Example notebooks are rerun and differences in results scrutinized
• Updated changelog

This PR addresses a failure in the clear-sky tamb function with pandas 1.3.0 (noticed in the eager-upgrade CI job on #281) and suppresses a bunch of deprecation warnings for casting datetime indexes to int dtype:

• AttributeError: 'numpy.float32' object has no attribute 'is_integer'
• FutureWarning: casting timedelta64[ns] values to int64 with .astype(...) is deprecated and will raise in a future version. Use .view(...) instead.

Note that the remaining astype/view warnings are from pvlib and have already been addressed on that end: pvlib/pvlib-python#1256

[cdeline]

I've tested this out and it seems to work, although I couldn't explain exactly how or why it works. I've seen other projects taking this approach to move from .astype to .view because it is suggested by the deprecation warning. I will say that using the .view function seems very kludgy and un-Pythonic, and I wonder if there isn't a different way to explicitly convert from datetime to epoch nanoseconds. All of the other changes you have here are just fine though.

[mdeceglie]

Though the pandas changes necessitating this are still somewhat mysterious to me, these changes all look good. Thanks @kanderso-nrel