Updating for compatibility with NumPy >= 2.0

[AustL]

With NumPy >= 2.0, some files raise the following error:

AttributeError: np.mat was removed in the NumPy 2.0 release. Use np.asmatrix instead.

For example, in MJ_solar_cell_tutorial. This should be fixed by replacing np.mat with np.asmatrix in all affected files.

[phoebe-p]

This has been addressed in the current development version (on the 'develop' branch), and the next release will be compatible with (and in fact require) numpy >= 2.0.