Moments of Maxwellians can be set from MHD equilibrium

Solves the following issue(s):

Closes #263 (closed)

Core changes:

• In the background parameters for Maxwellians, there is the new option mhd (instead of a constant value) for the moments:

    maxw_params_mhd = {'n': 'mhd',
                       'u1': 'mhd',
                       'u2': 'mhd',
                       'u3': 'mhd',
                       'vth1': 'mhd',
                       'vth2': 'mhd',
                       'vth3': 'mhd'}

If mhd is given, during the evaluation of the moments the mhd equilibrium is called and evaluated. Three evaluation methods are possible:

1. flat (marker) evaluation
2. phase space meshgrid
3. real space meshgrid (3D)

The perturbation is added on top of the MHD equilibrium. The broadcasting has been taken care of in the Maxwellian base class. Unit test have been added for Maxwellian6D and Maxwellian5D.

• Particle classes are now instantiated as follows (in StruphyModel._allocate_variables()):

                # background parameters
                if 'background' in val['params']:
                    bckgr_params = val['params']['background']
                else:
                    bckgr_params = None
                    
                # perturbation parameters
                if 'perturbation' in val['params']:
                    pert_params = val['params']['perturbation']
                else:
                    pert_params = None

                kinetic_class = getattr(particles, val['space'])

                val['obj'] = kinetic_class(
                    species,
                    **val['params']['markers'],
                    derham=self.derham,
                    domain=self.domain,
                    mhd_equil=self.mhd_equil,
                    bckgr_params=bckgr_params,
                    pert_params=pert_params
                )

In particular, we pass mhd_equil, bckgr_params and pert_params. In the Particles base class, there is always a self.f_backgr defined:

        # set background function 
        bckgr_fun = bckgr_params['type']

        if bckgr_fun in bckgr_params:
            self._f_backgr = getattr(maxwellians, bckgr_fun)(
                maxw_params=bckgr_params[bckgr_fun],
                mhd_equil=mhd_equil
            )
        else:
            self._f_backgr = getattr(maxwellians, bckgr_fun)()
            print(f'\n{bckgr_fun} is not in bckgr_params; default background parameters are used.')

The signature of the Maxwellians has been changed a bit, in order to make it more clear:

def __init__(self, maxw_params=None, pert_params=None, mhd_equil=None)

• Other changes:

• Rename toy model DriftKinetic to GuidingCenter
• Check argument shape and ndim in call method of Maxwellian base class.
• Added moment_factors to Maxwellians
• Add small offset to ScrewPinch equilibirum pressure to avoid zero values

Model-specific changes:

None

Documentation changes:

None