Add `CategoricalVector` and `collapse`#88

Merged

timholy merged 14 commits intoJuliaArrays:masterfrom

invenia:flatten

Aug 3, 2017

Commits on Jul 25, 2017

Merge branch 'master' into flatten
iamed2
authored

Commits on Jul 26, 2017

Merge branch 'master' into flatten
iamed2
authored

Commits on Jul 27, 2017

Allow passing in output array data type to flatten
iamed2
committed

Commits on Aug 3, 2017

Change all flat-related terminology to collapse
iamed2
committed