ENH: Implemented read_potential() for single element eam file form (aka funcfl format)

Changed read_potential() method in eam calculator to implement reading potential from single element eam files (aka funcfl format). Added an appropriate test. Testing potential added as a string in separate source file eam_pot.py. Testing potential taken from LAMMPS: https://github.com/lammps/lammps/blob/lammps-icms/potentials/Pt_u3.eam