Mathemagix

[Joris van der Hoeven <address@concealed>]

Hi all,

I rationalized the include system in the basix package.
Indeed, basix.hpp used to include several files at the end of the file.
This has been reorganized now, but may require a few changes of the form

        #include <basix/basix.hpp>
->
        #include <basix/exception.hpp>

I made these changes in my own files. Bernard might need to update
a few other files accordingly.

Best wishes, Joris