Add Octave support

Currently, mdbnomics only works with MATLAB. The main reason is that it requires the webread and webwrite functions to access the RESTful interface of DBnomics. Those versions are not available in Octave 5 and before.

They should be incorporated in the upcoming Octave 6. However, the following bugs may complicate things:

• https://savannah.gnu.org/bugs/?func=detailitem&item_id=56624
• https://savannah.gnu.org/bugs/?func=detailitem&item_id=58217