Fixed issues with the computation of material interfaces in HTG

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.

Backport: release