Add tests for magnetic

[jimnel]

Add tests for the magnetic functions:

• get_spins
• get_sisj
• structure_factor_from_sij
• get_basis

[rajarshitiwari]

@jimnel Once the example for pulser is merged. You can use that to write the test for the magnetic.

[pablosuvi]

@rajarshitiwari I left a comment on the PR 74 some time ago. Maybe it was missed. Test for the function test for get_sisj() function still failing in the file magnetic_test.py.