Simplify solver instantiation

[tlunet]

The NumericalSolver class is instantiated using all individual parameters required to create
a DaitcheParameters, which is not very practicle from a user perspective.
To avoid that, one could simply allow the NumericalSolver constructor to take a DaitcheParameters
as argument, for instance :

class NumericalSolver:

    def __init__(self, params, **otherParameters):
        assert isinstance(params, DaitcheParameters), "params must be a DaitcheParameters class"
        self.p = params

This would simplify passing other sets of parameters (in particular if more parameters are added).