Solver for the BTE in highly symmetric nanostructures

[mrm24]

I modified few files to generalize few procedures so they can be used in both bulk and nano.
The main changes are in bte_nano, which is almost a "copy" of the bte.f90; also I removed magnetic field from there.
I've also corrected a bug, as the tensors where not properly dumped into the drag files for the iteration 0.

[nakib]

Dear Marti,

Thanks for the pull request. Can you check if you have added the new source file that contains the bte_nano_module? The CI broke with the following error:

<ERROR> *cmd_build* Target error: Unable to find source for module dependency: "bte_nano_module" used by "app/elphbolt.f90"

[mrm24]

Yep, my fault I forgot to "git add" the new file. Now it should compile.

[nakib]

Now all tests pass. Merged it just now. I'll get in touch regarding creating a test (suite) for the nano-stuff.

Thanks a bunch for this important contribution!

Best,
Nakib