Update ConstantMassBalance kwargs (#362)
Update ConstantMassBalance kwargs (#362)

* made ConstantMB usable for CESM

* corrected kwargs in ConstantMassBalance

* Modify 2nd kwarg too and add docs

juliaeis/oggm,bearecinos/oggm,OGGM/oggm,bearecinos/oggm,juliaeis/oggm,OGGM/oggm,TimoRoth/oggm,TimoRoth/oggm,anoukvlug/oggm,anoukvlug/oggm