isnan only accepts floats

have to either check for the string type first or coerce to float first or it will throw a TypeError on 'nan'. I've chosen the latter.
reusable isnan that also checks the nan_set

a better form of the isnan? use of nan_set is debatable, I guess. this is at least reusable.
same isnan problem as in data_utils

isnan will TypeError on strings, so check more carefully. This time use the flexible_isnan added to data_utils
Merge pull request #51 from fiendish/master

isnan only accepts floats