MEX, Octave and Context API

Since building MEX through GitHub Actions [should now be possible](https://github.com/matlab-actions/), the next major release is planned to add a new MEX version of the bindings.

Much like what the current `CallLib`-based version, the functions from the C header are grouped by type. Enumerations, also generated from the header, complement that, so we can generate "driver" functions that include most of the boilerplate, copying the numeric arrays, and, additionally, the mapping from DSS.Error to MATLAB exceptions.

As a bonus, supporting MEX should enable Octave users to use DSS_MATLAB, as Octave [doesn't support `CallLib` yet](http://hg.savannah.gnu.org/hgweb/octave/file/555676697eef/scripts/help/__unimplemented__.m).

This new version will support the DSSContext API from DSS C-API, which should allow users to employ `parfor` (testing still required).