MULTREGT: Only apply multipliers to +face

[NB: Not merge ready]

With this commit a build time #define symbol is introduced to control
whether transmissibility multipliers are applied to negative faces when
resolving the MULTREGT keyword. With the definition:

   #define MULTREGT_USE_NEGATIVE_FACE 0

The transmissibility multipliers will *only* be applied to the positive
faces, that seem to be the Eclipse behaviour.