Don't cast to numpy.array when cleaning up raw data

Else fields with large string cause outrageous memory usage
modularize nan value conversion

extract from read_data_objects; make a stand alone function convert_nans; do convert_nans inside of read_model_data_from_csv
BUGFIX: use more general isinstance instead of type
Fix confusing bug where codes were being incorrectly cast to string
Merge branch 'data_munging'

Conflicts:
	crosscat/utils/data_utils.py