[NL] Refactor NonlinearSolver to support multiple solver strategies

Jörg Buchwaldrequested to merge
joergbuchwald/ogs:refactor_nonlinsolver into master

Refactoring to avoid blowing up NonlinearSolver.cpp too much with future extensions (like Eisenstat-Walker, TR and line search algorithms)

  1. Tests covering your feature were added?
  2. Any new feature or behaviour change was documented?
Edited by Dmitri Naumov

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