Bug when checking investmentdata for HydroReservoir from EMRP

[espenfb]

There is a bug (related to previous issue) when EMI-extensions checks the invesmentdata for HydroReservoir that lead to an error.

[JulStraus]

You mean Issue 46 of EMRP, solved for version 0.9 in #73 and for 0.8 in $74? I was apparently not sufficiently diligent in the fixes incorporated...

Considering the problems we received through allowing for different fieldnames, I regret to allow for .vol in HydroReservoir....