Geometry: Switch from hardcoded globalInsideP0_m to auto-detect

@gsell, the SAAMG solver still has a hardcoded varible globalInsideP0_m that should automatically be determined by the geometry class during geometry loading. Making it an OPTION could be an intermittent fix, but really the geometry handler should be able to determine a point that is clearly inside the geometry on its own.

from ArbitraryDomain.cpp:

    // TODO: THis needs to be made into OPTION of the geometry.
    // A user defined point that is INSIDE with 100% certainty. -DW
    globalInsideP0_m = Vector_t(0.0, 0.0, -0.13);

@frey_m how does the AMR solver handle this? Or is it not using an imported geometry?

Cheers, Daniel