Improved NaN filtering for DNI

(cherry picked from commit e69ecf32e1c0787998c476e1f806ce32aa11b4d8)

Conflicts:
	pvlib/clearsky.py
upkeep
Filtering modifications

Conflicts:
	pvlib/clearsky.py
Merge pull request #14 from wholmgren/develop

merge sandia cherry-pick via develop
Merge pull request #15 from UARENForecasting/master

merge sandia cherry-pick