Demonstrators for working with Surfaces in 3D

Motivated by the issue that led to [6b4c8e95] I tested the possibility to work with 3D surfaces in HyTeG. As it turns out this works at least partially. We can construct them, use them to generate function objects, interpolate expressions into the associated FE spaces and output the functions for visualisation correctly.

I added a tiny 3DSurfaceDemo app to show that and adapted the SPHdemo to allow computing the spherical harmonics only on a thin shell:

What I did not test so far is anything operator related, though.

Cheers
Marcus