Fix symbolic derivative computation in Monomial and restructure code to make it suitable to final integration into DCPSE.

1) The derivative computation in Monomial was totally wrong as it did not account for the scalar factor. Now it has been fixed and the tests modified accordingly.
2) The interface of Support has been changed to make it more suitable for the expected usage in the integrated DCPSE computation. Interfaces of the other classes interacting with Support has been changed accordingly.