Feature Request: Add SubFigures to Mpf_Figure

Nobody has claimed this yet.

Presently the Mpf_Figure class does not support mpl subfigure plots. Subfigures allow encapsulation of a group of subcharts into one figure that can then be embedded easily in the overall figure

Adding the following function to Mpf_Figure class will add in subfigures
``` Python
def add_subfigure(self,*args,**kwargs):

if 'style' in kwargs or not hasattr(self,'mpfstyle'):
style = _check_for_and_apply_style(kwargs)
else:
style = _check_for_and_apply_style(dict(style=self.mpfstyle))

sf = mplfigure.Figure.add_subfigure(self, *args, **kwargs)
sf.mpfstyle = style
self.subfigs += [sf]
return sf
```

Current Alternative solution / workaround is to sub-class Mpf_Figure and add this in. It would be preferable not to have to do this as SubFigure are part of the mpl and should be supported in mpf

Locate the Mpf_Figure class and its existing subplot or figure-management methods; the issue does not name a file or test. Compare those entry points with Matplotlib's Figure.add_subfigure API, then verify that subfigures inherit the relevant style, are tracked by the figure, and can be used without subclassing Mpf_Figure.

Written by the indexing model from the issue text.