Add methods for toggling geometry description mode
Add methods for toggling geometry description mode

Add following methods for toggling between description by regions and by planes:

- nslabs() gets total number of slabs in the geometry
- getIR(const int& z, const int& r) calculates region number from ring and slab numbers.
- getR(const int& ir) computes ring number from ir and nslabs()
- getZ(const int& ir) computes slab number from ir, getR, and nslabs()

Added method MGEOInputs::mapRegions() to swap between geometry description modes
by_regions and by_planes.

marenaud/EGSnrc,ftessier/EGSnrc,ftessier/EGSnrc,mchamberland/EGSnrc,crcrewso/EGSnrc,nrc-cnrc/EGSnrc,nrc-cnrc/EGSnrc,MartinMartinov/EGSnrc,mchamberland/EGSnrc,ftessier/EGSnrc,mchamberland/EGSnrc,marenaud/EGSnrc,marenaud/EGSnrc,ftessier/EGSnrc,nrc-cnrc/EGSnrc,mchamberland/EGSnrc,marenaud/EGSnrc,crcrewso/EGSnrc,nrc-cnrc/EGSnrc,MartinMartinov/EGSnrc,marenaud/EGSnrc,mchamberland/EGSnrc,marenaud/EGSnrc,crcrewso/EGSnrc,ftessier/EGSnrc,nrc-cnrc/EGSnrc,MartinMartinov/EGSnrc,MartinMartinov/EGSnrc,crcrewso/EGSnrc,crcrewso/EGSnrc,mchamberland/EGSnrc,MartinMartinov/EGSnrc,ftessier/EGSnrc