Allow embedding MatplotlibSparseMatrixViewer in another axes or figure

[`MatplotlibSparseMatrixViewer`](https://github.com/usnistgov/fipy/blob/master/fipy/viewers/matplotlibViewer/matplotlibSparseMatrixViewer.py#L23) creates its own figure. It would be nice if it could be embedded in an existing figure or axes.