This commit addresses to main issues:
1. The AxisReflection was /adding/ modified arrays, as opposed to
   modifying in situ the interface arrays (vectors and intercepts).
   This had initially made on purpose with the intent of temporary
   retaining the original arrays for debugging purposes. However,
   this had been forgotten and left in place. As a result, interface
   computations after a reflection where incorrect. This bug was
   not revealing itself in the test harness so far as this combination
   (reflection + interface computations) was not exercised.
   As a result, this commit addresses the issue but also verifies
   this combination. Furthermore, the HTG source was modified in
   order to allow for the specification of intercepts arrays in
   the form that exercise this configuration.
2. The Geometry extraction was not building material interfaces, as
   a result of some code having been leftover during the HTG-v2
   merger in VTK 8. This commit fixes this issue, and also adds
   a unit test to verify material extraction together with its
   corresponding non-regression image.