REF: refactored convertToPix() to use a dict of functions (Sol's idea)

Should improved extensibility and reduces a potentially long if...elif sequence
ENH: added 'degFlat' and 'degPosFlat' units options for stimuli

These accomodate flat screens for both position and size/ori, or just
for position (centroid).

Also fixed bug in previous commit (private names rather than public)
DOCS: changelog
FF: forgot to import numpy for monitorunittools