Update interstitial class

[samwaseda]

Following the spontaneous meeting between @pmrv and me, I updated the interstitial class, in the hope that it does what it's supposed to do now.

Main changes:

• Now the workflow is more easily accessible
• Classification takes place following the Steinhardt parameters

[jan-janssen]

@samwaseda Can you add tests for this functionality? To me it seems the functionality changed but all the tests pass, so maybe it is not sufficiently tested.

[samwaseda]

It tests the full functionality instead of individual functions, which is the reason why it doesn't fail, but ok I can also add individual tests.

[pmrv]

I left some docstring nits, especially for cluster_by_steinhardt, because that's non-obvious for other devs. Otherwise lgtm.

Tests are ok imo as they check that known interstitials are found in simple crystals.

[pmrv]

Ah also I forget: This removes the iterative "refinement" of interstitial positions, in the last session we never used it anymore, but are there cases where it still might? I think you mentioned bcc and that's covered by the tests so might just be remembering it wrong.

[samwaseda]

Ah also I forget: This removes the iterative "refinement" of interstitial positions, in the last session we never used it anymore, but are there cases where it still might? I think you mentioned bcc and that's covered by the tests so might just be remembering it wrong.

It still does the refinement, but it's not necessary to set it anymore because the algorithm checks the convergence automatically.