Return value for the method "periodic_table_heatmap" should change from "plt" to "fig, ax"

[hezhengda]

https://github.com/materialsproject/pymatgen/blob/b5eda8d3c343a6cb06c5cc02a34dbd30158637d1/pymatgen/util/plotting.py#L186-L329

If "plt" is returned, there is no object in it, only a class (matplotlib.pyplot) will be returned.

If we change the return to "fig, ax", then the periodic_table can be export by fig.savefig(filename, dpi..).