Refactor problem classes

[maxpaik16]

The problem class structure can be refactored and made more clear. Many specific problems (for example, ElasticProblems and NodeProblems) are unnecessary as json files can be used instead to create an equivalent GenericProblem. This refactor will also allow certain parts of the code that only affect one type of problem to be simplified, such as in State::build_basis and State::assemble_rhs.