Fixed another bug in the AERONET import routine.

It used to crash when the nearest date/time had any NaN values for
important data (eg. Refractive Indices). It now gets the nearest time,
checks for NaNs, and if there are any, gradually works its way down
the 'nearest indices to the given time' list until it gets a row without
NaNs.
Added more detailed error messages for failures during AERONET data import.
Fixed bug where geometry parameters were only being specified as ints.

This was affecting all specifications of latitude, longitude and decimal GMT time in Geometries.
Merge branch 'master' of github.com:robintw/Py6S

Conflicts:
	Py6S/Params/geometry.py
	Py6S/SixSHelpers/aeronet.py