Too many sympy symbols imported

[kpeeters]

We should not import any sympy symbols into the global namespace, to avoid people calling sympy functions on Cadabra expressions and expecting that to work in all situations. We then need wrappers in cdb.sympy.*. See also https://cadabra.science/qa/2086/diffrentiation-of-an-expression.

[kpeeters]

There is now a cdb.sympy.calculus package with a properly bridged diff function. We are still importing too much though, so it is still possibly to inadvertedly call the sympy function if you forget to import the one from the package above.