Updated visualization for single lens.

Changes:
- Updated to use new mainWFS.py which has the lenslet array for the numerical solution
- Improved plot aesthetics
- Removed SingleLens.py which is no longer needed
- Attempted to improve readability and modularity
I'm not sure why you get them sequentially.  I thought that calling the pl.show() only at the end would display all three together (which is what I see here).  Could it be a difference in how we compile the code?  I am using the command line directly, are you using a compiler?