Rename matrix_type labels in metadata

_From @ecsalomon on September 11, 2017 15:36_

This is related to supporting "inspections" and "eis" pipelines, #39

This change should also rename the `matrix_type` labels in the metadata to describe the actual difference between these kinds of matrices. Currently, matrices with rows only when there are labels get the `train` matrix type, and matrices with rows regardless of label are called `test` matrices. We come up with a sensible name that actually marks this difference—for example `no_unlabeled_rows` and `unlabeled_rows`.

This would also improve the efficiency of the computation time and storage, as the architect would recognize that a train and test matrix defined by timechop that share all time configurations *and* both include only labeled rows will not be built and stored twice.

_Copied from original issue: dssg/architect#40_