NF: support for "keys" argument when quering mask of selected features
TST: unit test for setting keys in the mask
ENH+TST: more unit tests
RF: use maximum 79 characters per line
DOC: clarify intent of masking using a list of keys
DOC: make more consistent with other doc strings
Merge pull request #232 from nno/_nf/qe_mask_keys

 nf/qe mask keys
ENH+DOC: use single-line docstrings
Merge branch '_nf/qe_mask_keys'