Incorporate internally consistent probe and source coordinates

[espenhgn]

Is your feature request related to a problem? Please describe.
At present, the different classes rely on different parameters names declaring extracellular recording sites:

• PointSourcePotential and similar uses ndarrays x,y,z in Cartesian coordinates,
• OneSphereVolumeConductor uses ndarray r in spherical coordinates,
• FourSphereVolumeConductor uses r_electrodes in Cartesian,
• InfiniteHomogeneousVolCondMEG uses sensor_locations etc.
• For class InfiniteVolumeConductor the user must specify relative measurement location r as input to the get_transformation_matrix,
• NYheadmodel something else altogether

These internal inconsistencies needs to be resolved and conform to the same structure, e.g.,

class Whatever(LinearModel):
    '''Some description
    
    Parameters
    ------------
    cell: CellGeometry object
    r_probe: ndarray
        shape (n_contacts, 3) array of extracellular measurement locations in absolute Cartesian coordinates (µm)
    '''
    def __init__(self, cell, r_probe, **kwargs):
        super.__init__(cell=cell)
        self.r_probe = r_probe
        ....

In a similar vein the source locations in case of dipoles should be declared with the same parameter, e.g., r_source.

A good timeline to incorporate would be for future LFPy-2.3 (https://github.com/LFPy/LFPy/projects/2) which may break backwards compatibility anyway.

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