Merge pull request #97 from astrofrog/fix-api-links-docstrings

Internal class references on doc pages should be links
better type and dimension checking for indices in structure_at
Merge pull request #98 from keflavich/structure_at_indexing

Minor change: check type of index before using it
Merge branch 'master' of github.com:dendrograms/astrodendro into multiple_selections_clean