Use ExtData2G to persist emissions

[acollow]

Currently, we only have CEDS emissions and MERRA-2 GMI data through the end of 2019 and have created files with future dates to run for subsequent years. With the capability for climatological persistence in ExtData2G it will no longer be necessary to store duplicate emissions files with future years in the filename.

[mathomp4]

Pinging @bena-nasa to advise on this.

[acollow]

@mathomp4, no need to fear! I was in touch with @bena-nasa regarding this and already implemented it. I am in the final stages of testing and hope to submit the PR later today or tomorrow.