Reflective interfaces

Addition of "special interfaces" for @lothar.lilge

This merge request is intended to show the user interface and permit initial testing. A "special interface" is a boundary between regions X and Y where the normal interface handling is overridden. Boundaries are marked using TetraKernel::markRegionBoundaryForSpecialInterface(x,y,r,a) method (also wrapped for Tcl scripting). At the interface, the following behavior is modeled for physical photons:

• reflect with probability r
• absorb with probability a
• transmit (including refraction) with probability t

Which is implemented as logic that causes photon packets incident on the interface to reduce their weight to w=w0*(1-a) and either:

• reflect with with probability r/(1-a) or
• transmit (including refraction) with probability t/(1-a)

Remaining items to do:

• Complete regression test
• Update documentation
• Finish analysis of test runs
• Update conservation scorer/checks