[python][fvproblem] add empty pointSourceMap to C++ wrapper
What this MR does / why does DuMux need it:
With !3829 (merged) the assembler uses problem.poinSourceMap
. This MR adds an empty map to the C++ wrapper for the python fvproblem.
To make the map usable in python problems further work is needed.
Edited by Timo Koch