Optional splitting of OBS flows into IN and OUT

[cneyens]

Is your feature request related to a problem? Please describe.
When using the OBS functionality to track flows to/from boundary conditions, only the net flow is written. For some cases, it would be useful to have both the inflow and the outflow per obsname, e.g. a series of RIV cells corresponding to obsname X can be gaining as a whole, but there might be some leakage into the aquifer.

Describe the solution you'd like
An optional keyword in the OBS input file for the boundary condition package which will divide the observed flows into inflow and outflow. The output file would have an additional column for each registered obsname. Suffix _IN and _OUT would denote the in- and outflows (similar to the global budget output file).

Describe alternatives you've considered
Getting the in- and outflows from the cell-by-cell budget file. This requires obtaining the corresponding cell id's for each obsname or boundname, which requires some additional work.