Convert CreateDescriptionFromTriangulationHelper into a function.

[bangerth]

In tria_description.cc, there is a class called CreateDescriptionFromTriangulationHelper that in essence consists of a constructor that sets a bunch of member variables, and a single public member function that takes some more arguments and then does something. This all can be handled in a more traditional way by just having a single free function that does the same thing -- it is easier to reason about where which data lives and who changes it.

This patch does this kind of conversion. The only marginal difficulty (other than the fact that the patch is likely hard to read because everything moves around) is that the constructor of the class calls a GridTools function to initialize two expensive members that can then be re-used over multiple calls to the one function in a loop. I handled that by moving this call to GridTools out to the calling sites, and letting them pass the result as an additional argument to the now free function.

The patch might be easiest to read without showing whitespace changes.

[bangerth]

Ping?

[bangerth]

Ping?