RemoveKeyVal - should delete key-value annotations and support HCS plates

[will-moore]

#191 (comment)

RemoveKeyVal: Strips off all key-val pairs in the special namespace from selected items. Issue is that it does not really remove the annotations (as in "delete from the database") but somehow hides from the view. They resurface under certain conditions like applying KeyVal_from_csv again. This one should stay and be debugged IMHO.

Also, mpievolbio-scicomp#1 (review) should support HCS plates.