Include the ViennaGrid source code in the repository / tarballs

[genba]

We must think of a better way to include the ViennaGrid source code to make compilation easier.

Currently, the source code of ViennaGrid, on which ViennaGrid for Python depends, is included in the repository as a Git submodule. This is fine if the user gets and compiles the code from the repository, because they can also fetch the ViennaGrid code either by cloning and updating the submodules:

git clone https://github.com/genba/viennagrid-python.git
cd viennagrid-python/
git submodule init
git submodule update

or just by cloning with the --recursive option:

git clone --recursive https://github.com/genba/viennagrid-python.git

and they will be able to compile ViennaGrid for Python.

However, if the user downloads a tarball, the Git repository information will not be included in the tarball, and thus no submodule information will be available. In other words, the user will have to download ViennaGrid by themselves.

The script bootstrap.py intends to aid the user in this task, but it currently only updates the submodules; it doesn't download ViennaGrid separately. This must be fixed, so that the script download ViennaGrid separately when no Git repository information is available.

Besides, other approaches could be search, for example, with the ExternalProject_Add command of CMake.