Add model reader for gocart output

[MaggieMarvin]

These changes include the addition of a model reader for gocart output that can be used to process GEFS aerosol files. As written, this code reads in total AOD at 4 wavelengths (470, 550, 670, and 870 nm) and currently drops/ignores the speciated AOD variables generated by gocart (AOD_BC, AOD_DU, AOD_OC, AOD_SS, and AOD_SU), but could be extended to include those variables if they're likely to be used regularly.

[zmoon]

@MaggieMarvin do you have an example file or two so we can write a test?

[MaggieMarvin]

@zmoon Great, thanks! Yes I've tested in melodies-monet - the model files I used are here on Hera: /scratch2/NCEPDEV/stmp3/Margaret.R.Marvin/global/stmp/RUNDIRS/WetDep_Dust/gefsfcst.2018081600/fcst.371442/gocart.inst_aod*.nc4

[zmoon]

I extracted a small spatial subset of one of the files to use for test. gocart.inst_aod.20180901_1200z.subset.nc4.zip