Non templated problem

[fdarricau]

Work in progress:

• First step: the type-checking system
  • setting the flags
  • creating the architecture of methods needed to provide a runtime typing comparison
  • writing tests
• Last step: Problem<EigenMatrixDense/Sparse>
  • Revamping problem.h* and solver.h*
  • adapting their tests

Once over, every plugin and dependent modules will have to be updated accordingly

[bchretien]

No need for a trailing underscore for static parameters, since you call it in a public way, e.g. GenericDifferentiableFunction<T>::flag_.

[fdarricau]

Ok so now that the Problem and Solver classes are rewritten, we will probably have to provide an "automatic_cast" that uses the flag system to retrieve the actual function.
We could even rewrite the cast_into so that it automatically gives the last one.

Three tests fail, and it is basically because we can't check the constraint type as before. So something able to retrieve the actual function type would solve it.

[bchretien]

Another idea for the enum: to prevent clashes, we can also wrap it in a struct, cf. what I did there.

Also, this PR should not be on master since this is a huge API change. This should be on a separate branch until we give the all clear.

[fdarricau]

Indeed, it would be cleaner to base the PR on a related branch, but none of the active branches is really related to it (and many of them are quite old, so the merge process might be complicated)
Can you create one ? Then I will move it there

[bchretien]

I guess we can create a dev or v4.0 branch.

[fdarricau]

Moving this to a new PR