FF: various fixes to degFlat and degFlatPos unit conversions picked up by new tests

- make sure that pos and vertices are numpy arrays
- make sure that returned vertices have datatype 'd' (not 'f') or shapes go wrong
- cleaned up code for using these units with Text and SimpleImage, which had
additional references to unit code in their bodies
TESTS: Added tests for the new window units, 'degFlat', 'degFlatPos'

Also fixed minor discrepency in returned fit params for naka-rushton