[Python] fix ProjData_read_from_file

[KrisThielemans]

stir.ProjData_read_from_file is not available on recent SWIG (4.1) unless you use a specific switch, see https://swig.org/Doc4.1/Python.html#Python_nn20. So use stir.ProjData.read_from_file

[robbietuk]

Looks good to me.

Can the swig compiler detect earlier swig versions? If so, could the stir.ProjData_read_from_file function wrap a stir.ProjData.read_from_file call? It would streamline a lot of code and make it SWIG version invariant.

[KrisThielemans]

Can the swig compiler detect earlier swig versions? If so, could the stir.ProjData_read_from_file function wrap a stir.ProjData.read_from_file call? It would streamline a lot of code and make it SWIG version invariant.

Not really I guess. We can do it in our own .i file, and add the function. However, I wouldn't bother. The . syntax is much more natural, and there already a long time, see e.g. in 3.0
https://swig.org/Doc3.0/Python.html#Python_nn20. I guess you might have had problems with my MATLAB swig branch, which is quite out-of-date...