Add retrieval function for NASA MERRA2 reanalysis data

[AdamRJensen]

• [ ] Closes #xxxx
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MERRA-2 is the reanalysis dataset provided by NASA and provides hourly aerosol and irradiance data for the entire world starting in 1980. While the irradiance data is of questionable accuracy, the aerosol data is very useful for a number of different clear sky models.

The proposed function retrieves MERRA-2 data from the NASA GES DISC repository.