Force test using finite differences

[PicoCentauri]

Currentlty, we test our forces against references forces computed with GROAMACS:

torch-pme/tests/calculators/test_values_ewald.py

Lines 225 to 237 in 3540a81

	def test_random_structure(
	cutoff, frame_index, scaling_factor, ortho, calc_name, full_neighbor_list
	):
	"""
	Verify that energy, forces and stress agree with GROMACS.
	Structures consisting of 4 Na and 4 Cl atoms placed randomly in cubic cells of
	varying sizes.
	GROMACS values are computed with SPME and parameters as defined in the manual:
	https://manual.gromacs.org/documentation/current/user-guide/mdp-options.html#ewald
	.. code-block:: none

Additionally, we should add tests that are independent from a references for the forces using simple finite difference tests.