Merge pull request #83 from astrofrog/fix-get-mask

get_mask should default to shape of generating image
Merge pull request #72 from astrofrog/enforce-single-warning

On some platforms, warnings are repeated many times
Merge pull request #80 from astrofrog/deterministic

Ensure that results from repeated runs are deterministic
Merge pull request #70 from astrofrog/stats-area

Add area to available statistics
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