Added getNXYZ() to the grid property.
Added MULTNUM keyword
Updated FaceDir functionality to support MULTREGT
Removed dummy -1 default values from MULTREGT
Added class MULTREGTScanner

The class MULTREGTSCanner internalizes the content of one or several
MULTREGT keywords and can then scan through GridProperty region objects
to find region interfaces as specified in the MULTREGT keyword.
Added MULTREGT integration test.