Reduce t8code compile time for developers

[sandro-elsweijer]

t8code has become rather big and needs really long to compile. Especially the introduction of templates introduces some significant overhead and it would be nice to add some flags to cmake to make some modules optional. We can still make it default to build them, but we as devs could deactivate them.

These modules could be made optional:

• Default scheme
• maybe some vtk parts? (@Davknapp)
• some more complex geometries

[Davknapp]

See #1547 for making the vtk-module optional. Ascii-vtk will also always be build in that branch. Furthermore we are working on building netcdf and occ parts of t8code only when they are enabled in the cmake-options.

I like the idea of not always building all geometries!

[spenke91]

AMR meeting: Maybe CMake modules might be interesting / helpful?