Restructure

[dschick]

this is a first attemp to restructure pyGTM and to separate the GTM formalism from the actual structure parameters and geometry.

The tutorial in the example folder should work and generate the same figues as with the old code.
right now the code does not save the all the matrices in as object attributes which makes the actual structure much more lightweight.
On the other hand the code needs further improvements to avoid multiple calculations of the same matrices. This can be easily done by revising the for loops.

More efforts need to be done for the import scheme to simplify the usage from within a python script or jupyter notebook.
I can propose something here if the current structure is satisfactory.