Implementation of the DFT derivative

[rhodrin]

In another branch I've added derivative functionality but the implementation is currently horrible.

I think the interface (Solution dsol = sol.diff(n);) is fine but diff being a member function of Class Solution leads to some nasty stuff. Currently dsol is computed in Solution.cpp which it shouldn't be, but introducing a new, i.e., DFTDerivative class leads to some circular dependencies.

From some Fixme comments I gather you were thinking of doing some restructuring of the Solution class anyway? If you've got any suggestions on how you think the derivative code should be structured please let me know!

[ehkropf]

I'll have a think about this over the weekend. Thanks.

[ehkropf]

I think this can be done by

1. adding a derivative() method to ClosureInterpolant (and related classes),
2. and then making Solution a ClosureInterpolant.

I think I meant to make more use of ClosureInterpolant, but didn't quite get that refactoring done.

[ehkropf]

Solution is now a ClosureInterpolant as of #21. Remaining: adding derivative() to

1. RealInteroplant,
2. ComplexInterpolant, and
3. ClosureInterpolant
   in that order.

[ehkropf]

I think everything is in place with f594aad (dev branch) except for the actual modification of the DFT values and the derivative test functions. All real derivative work is done in RealInterpolant (see the TODO notes in derivative() there).

[rhodrin]

Thanks. Will take a more detailed look this week.