bugfix: time handling for GEOS-5 aerosol surface data

GEOS-5 2-D surface data is identified by geos2wrf by parsing the filename of the input files. However, the method implemented assumed that the files were in the current directory and did not contain any path information ahead of the filename. This caused the times to be parsed incorrectly for the surface emission data and they were getting placed into a different time offset by an hour. This has been corrected by changing the way the filenames are parsed--we now count backward from the end of the filename instead of from the beginning of the filename to search for "tavg3".