ENH: Add a Vasp locpot class/reader

Added the class VaspLocpot to ase.calculators.vasp.vasp_auxiliary. This will read in a LOCPOT file from VASP. Two additional functions were written which will return the average along a specified axis and the other function will return the scalar distance along a specified axis. A good portion of the code was borrowed from the VaspChargeDensity class in the same file.