Try and clarify situation about ``idx`` not being deterministic
Attempt to define a deterministic structure ID
Refactor how the smallest index is found, and re-assign the idx of structures
Make sure ``_structures_dict`` uses the new IDs
Change null value for idx to be -1
Updated docs
Fixed failing test
Updated another instance of the index_map
Updated structure IDs in plotting examples
Revert another change in the docs
Merge pull request #84 from astrofrog/clarify-idx

Encourage to split compute and analysis and make clear that structure IDs can change at every compute