Include grammar in the package

The grammar files are in the top-level folder grammar of the repository. So far it is accessed by the scripts from scripts folder, by the tests from tests folder, by the Jupyter kernel from virtmat_kernel folder, all outside of the main virtmat package which is in /src. Now, the grammar must be accessible from the package files (needed in MR !150 (merged)) but currently it is not included itself in the package. A non-editable install ends up with an installed package without the grammar files. The grammar then cannot be found on the system except in the git repository that may not be available at run time.

The grammar files can be interpreted as data files according to the setuptools docs. To achieve this we have to move the grammar folder to src/virtmat/language and configure it in the setup.cfg file and also fix the other grammar entry points under scripts, tests and virtmat_kernel.