modified add operator to enable composit lenses.  Adds kappa maps and deflection angles for each pixel.  Current restrictions:  Array dimensions and pixel scales of the two lenses must be the same. Works Towards #17
modified sub operator so that differences in kappa,alpha between two maps can be estimated.  Probably other uses also.  Limitation:  Can only be done with kappa maps of same shape and pixscale. Fixes #17
New ipython notebooks have more plots and tests.
making sure this branch is up to date.
fixed unit problem that was giving einstein radius in radians instead of arcsec.
source.py now is more flexible with rectangular images converted to FITS from JPEG.  JPGconverter.py now has pixel sizes that correspond to the images in the lensed image test (can be made more general in the future).
conclude merge.