- diff() is now only defined on classes ex and basic, where it handles

- diff() is now only defined on classes ex and basic, where it handles
  higher derivatives.  derivative() is now the method to be implemented
  by the user that does single derivatives, it is protected.
- Implemented some very clumpsy automatic generation of ginaccint.
  It is ugly because we have to compile libginac without namespace GiNaC.
- Fixed some wrong logic in numeric::power().