include GLOBGM as a module

[edwinkost]

Description
Include GLOBGM (the MODFLOW groundwater model of PCR-GLOBWB) as a module.

Problem
Although it is known that PCR-GLOBWB can include a MODFLOW groundwater model (e.g. since Sutanudjaja et al. 2011 to Verkaik et al., 2014), we have not really formalized this feature yet in https://github.com/UU-Hydro/PCR-GLOBWB_model.

Implementation
Edwin will try to merge the latest developments in the respect of GLOBGM, more specifically from https://github.com/UU-Hydro/GLOBGM and https://github.com/edwinkost/PCR-GLOBWB_GM-GLOB (e.g. https://github.com/edwinkost/PCR-GLOBWB_GM-GLOB/tree/modularizing)

Downsides
None, I guess.