Add thermodynamic potentials

Not ready to be merged. Opening for comments/suggestions.
Open the file once and create a dictionary element->mass. Then only look up in there.

Something like:

    lookup=None

    def mass(el):
    if lookup=None:
      lookup = dict()
      for line in open(filename, "r"):
        data="%".join(line.split("%")[:1]).split()
        lookup(data[0])=lookup(data[1]))

    return lookup(el)