Merge the dune-tnnmg-examples module into dune-tnnmg
Hoping that the example and the text from dune-tnnmg-examples will get more attention and rot less if they are in the main module.
WIP because right now the MR is almost a verbatim copy of the code in dune-tnnmg-examples, and nothing builds and nothings runs. Will continue.
One problem I didn't think about when I started copying: The examples code need many more Dune modules that dune-tnnmg itself. Currently they need at least dune-functions, dune-typetree, and dune-pdelab. I suppose we have to add them as optional dependencies. Is everybody okay with that?