Current limitations on cell rotations and translations

[GiudGiud]

While PR #395 fixes the major problems with cell rotations and translations, it doesn't address everything. I'll leave in this issue what has not been addressed for now, and report any progress.

• cell.getMinX-Y-Z and getMaxX-Y-Z don't take into account cell rotations. Because using only axially-extruded geometries limits what rotations can be done, this is mostly an issue if a cell is rotated along X or Y by 90 / 180 / 270 degrees.
• getMin/Max XYZ on a XYZ Planes rotated by a non-right angle (not 90, 180 or 270) should return infinity, so a useful routine should look at the intersection of two planes to return the right answer. This is very far from the current abstraction
• the respective routines to find boundaryTypes at minX-Y-Z and maxX-Y-Z also have this issue. This can be a more serious problem, if a geometry has a boundary condition that involves a rotated cell. This should be pretty uncommon though.

So overall, be careful and plot your geometries / check with OpenMC if you are using geometries with cells defined using rotations and translations.