Raise error when saving bad variable name to mat file

[valentinsulzer]

If a .mat file has a variable name with a space in the name, MATLAB won't be able to load it. So we shouldn't save it. Raise error instead and suggest making a new variable with a more MATLAB-compatible name.
We could try converting spaces to underscores, etc, but units ([mol.m-3]) would not work (none of [, ., - are ok)

[valentinsulzer]

Alternatively, add an extra keyword argument to solution.save_data which allows the user to provide a dictionary of name conversions

[rtimms]

I think passing a dict of name conversion is more user-friendly that creating new variables. I guess we still raise the error and then suggest passing the dict