DetDesc - 'Solids' classes cleanup

Minor cleanup / modernisation of 'Solids' classes in DetDesc.

I started off looking to see if I could speed things up at all, as the intersectionTicks method is an increasing percentage of the CPU time spent in the RICH, as it is used during the track treatment to find the radiator volume entry/exit points. In the end I did not find much, perhaps O(1%), primarily from refactoring a few methods to help with inlining etc.

@cattanem Might not be 100% bit wise identical to before. Obviously though, should not be any significantly different.