Add the jupyter kernel to the package

Currently, the jupyter kernel is in the virtmat_kernel/ folder and not under src/ where all files for the actula package should be. Therefore, the kernel is only available in a clone of the git repository. The kernel has to be moved to src/virtmat/language and an entry point can also be provided.

The issues #148 (closed) and #149 (closed) might also be solved by this move.