Provide examples structures in main code

[PicoCentauri]

@ceriottm suggested in #82 (review) to add a data module to provide a few example structures like crystals that we are using for the tests. The code to create these is already there inside the testsuite

torch-pme/tests/helpers.py

Lines 13 to 18 in 7aea00f

	def define_crystal(crystal_name="CsCl"):
	# Define all relevant parameters (atom positions, charges, cell) of the reference
	# crystal structures for which the Madelung constants obtained from the Ewald sums
	# are compared with reference values.
	# see https://www.sciencedirect.com/science/article/pii/B9780128143698000078#s0015
	# More detailed values can be found in https://pubs.acs.org/doi/10.1021/ic2023852

I am not sure if I would put this function inside the main library because seems not really the scope of the torch-pme to provide crystals. There are much better third party tools like ase to get these.