ENH  dimensions:  allow None as ndvar.subdata() argument value;  add Sensor.index() to find index minus bad channels
ENH: rename  isscalar() -> isnumeric(); add as numeric()
ENH process.mark_by_threshold():  handle bad channels;  FIX capitalized parameter name
cleanup:  dimensions
RF:  merge dimensions into data module
cleanup dimensions:  Sensor.getLocs2d --> get_locs_2d;  Sensor channel name to index conversion;  proper pickle support for all dimensions