Add magnetic quadrupole and octopole point sources

[leouieda]

The magnetic quadrupole and octopole are useful point sources in magnetic microscopy. Their fields can be calculated using the third and fourth order (respectively) derivatives of the 1/r function. It would be useful to have them in Choclo. The maths can be found in this expanded abstract https://doi.org/10.1071/ASEG2006ab064 and Chapters 7-9 of this PhD thesis: https://digital.library.adelaide.edu.au/items/880eac51-cffc-4e7e-84cb-3ef488b8ebc1

The quadrupole can already be added after #134 but it would be good to get kernels for the fourth order derivatives before attempting octopoles.