Simplify Operate by using get_int_value() instead of to_python()

get_int_value() handles conversion to Python and checking that the
input is the proper type. This means we only need one pattern.
Operate: improve message and add test case for n < 0
Use SymPy to improve Variables

This makes it match the first few examples in the Mathematica docs,
but it's still not complete. Add a note about this.