Update usage_notes.md
put spheral module under sph namespace and added pext pmin pmax to tillotson eos
	modified:   SpheralPlayground/equations_of_state.py
added pext, pmin, pmax to tillotson demo and added detailed header to pflatten
	modified:   MATERIALS.md
	modified:   MATERIALS.md
Update MATERIALS.md
Merge branch 'master' into develop