Update city emissions scripts

The emissions data grid class has been generalized for any emissions dataset. A new CMIP5 class was extended from the general grid class to handle CMIP5 specific transformations. New datasets should follow suit.

This takes better advantage of the Pandas time series functionality, but most calculations are still done in numpy because they are orders of magnitude faster for this dataset. (At least compared to the given python version.)

The CMIP5 gridded emissions data has been added to the repo, however, because ORNL has a 1GB file size limit, it was split into two files. You'll need to combine them before running any of the python scripts using data/combine_CMIP5_emissions_dataset.sh.

All output and file names have been improved and should be much clearer.

fixes #2 (closed), fixes #3 (closed)