plot_windmatrix fails if source == None

[jamesramsden-bh]

It is possible to create a Wind object with the source unset, in which case it defaults to None. This is not an issue in itself, but plot_windmatrix fails if self.source == None, as line 1679 attempts to append to a string:

        title = self.source
        title += f'\n{kwargs.pop("title", None)}'

Please ensure that NoneType is handled when creating the plot title here.