Implement Symbol::get_name()
Allow to convert Symbol to SymPy and test it
Implement Mul::_sympy_()

We exposed Mul::as_two_terms() and use it to do the conversion.
Add tests
Implement Pow::_sympy_()
Improve tests
Implement Rational::_sympy_()
Implement Integer::_sympy_()
Improve a test
Implement Add::as_two_terms()
Implement Add._sympy_() and test it
Improve tests
Convert sin/cos as well
Add a note
Start implementing conversion to SymPy
Install sympy when testing Python
Convert sympy.Mul
Convert sympy.Add
Convert Pow
Convert Integer and Rational