Adding pretty print and reconstruction

``Pretty print'':

class StringFunction:
    ...
    def __str__(self):
        return self._f  # just the string formula

Reconstruction: a = eval(repr(a))

    # StringFunction('1+x+a*y', 
                     independent_variables=('x','y'), 
                     a=1)

    def __repr__(self):
        kwargs = ', '.join(['%s=%s' % (key, repr(value)) \
                     for key, value in self._prms.items()])
        return "StringFunction1(%s, independent_variable=%s"
          ", %s)" % (repr(self._f), repr(self._var), kwargs)