Additional TrendAnalysis tests for coverage

[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

Adds some tests to get analysis_chains.py coverage up from 91.3% to 96.7%. Current overall coverage:

Name                              Stmts   Miss  Cover
-----------------------------------------------------
rdtools\__init__.py                  21      0   100%
rdtools\aggregation.py                3      0   100%
rdtools\analysis_chains.py          276      9    97%
rdtools\availability.py             158      0   100%
rdtools\clearsky_temperature.py      78     10    87%
rdtools\degradation.py              117     10    91%
rdtools\filtering.py                 17      0   100%
rdtools\normalization.py            234      6    97%
rdtools\plotting.py                  94      0   100%
rdtools\soiling.py                  274      6    98%
-----------------------------------------------------
TOTAL                              1272     41    97%

[mdeceglie]

Looks good, thanks @kanderso-nrel