Coerce mef.get_transform_fxn input to permit None values.
Coerce mef.get_transform_fxn input to permit None values.

Coerce `mef_values` input to mef.get_transform_fxn to be a numpy
array of floats. This coercion converts None values to np.nan,
thereby permitting None values as synonym for np.nan. The coercion
also eliminated the need for some type checking.

taborlab/FlowCal,taborlab/FlowCal