Small changes for integration of Abs

- rewritten as Abs as Heaviside
- rewritten as Heavisides as sign
- integrals with simple heaviside terms
- added ExprCondPair.is_polynomial
`heavisideintegrate(x*Heaviside(y), x)` fails, if we remove the test for `m==0`.  But if that's handled elsewhere, I will remove the if-then.