to_sympy(String) returns None; use repr for to_python()

[rocky]

in core/expression.py:

class String(Atom):
    ...
    def get_string_value(self) -> str:
        return self.value

    def to_sympy(self, **kwargs):
        return None

    def to_python(self, *args, **kwargs) -> str:
        return '"%s"' % self.value  # add quotes to distinguish from Symbols

to_sympy() should probably return self.value. Also I think there shoudl be a parameter to to_python() that indicates whether you want quotes or not. And to_python() is wrong since repr should be used to handle the fact that the string might have embedded quotes.