FEAT: add skeleton for amico implementation

[matteofrigo]

With this PR we add the skeleton for the implementation of AMICO. Let's open the discussion @Sara04 @rutgerfick .

dmipy.core.modeling_framework

For the MultiCompartmentAMICOModel and the MultiCompartmentAMICOSphericalMeanModel we have to do the following:

• Implement _create_forward_model_matrix
• Choose a strategy for the solver to use. The difficulty stands in keeping the syntax consistent with the other models
• Update documentation of fit function
• Complete the setup of the optimization before model fitting in the fit function, taking into account that the look-up table must be built or loaded.
• Implement the fit_func method in the fit function
• Preprocess the x0 as required by the solver
• Implement the multi-tissue feature correctly

dmipy.core.fitted_modeling_framework

• I added the fitted_distribution property to the FittedMultiCompartmentAMICOModel and FittedMultiCompartmentAMICOSphericalMeanModel classes. This is intended to return the distribution of each parameter obtained from the AMICO fitting.
• The fitted_parameter property could be built on top of the fitted_distribution.