Update to basic utilities.

Separated basic utility (math - in pImage.py) from plotting (in
pImagePlots.py) from creating test data (in testImage.py). Fixed small
bug in testImage.py that would make non-square images fail. Added
capability to invert FFT, PSD, and/or ACF (and 1d versions of PSD and
ACF) into Images (!!). Added calculation of structure function.
Updated test.py for changes in basic classes, and added testGaussian.py

testGaussian.py walks through the evaluation of a gaussian in
Image/FFT/PSD/ACF space, with comparisons to the analytic expectation.