graphics/waveforms.py refactoring

[rmodrak]

Trying to add the following

• better handling of empty datasets and other special cases
• testing coverage (tests/test_graphics.py)
• improved code readability

Tentatively aiming to merge after Dec 14

[thurinj]

Hi @rmodrak ,

I need to check your branch to test it out. Looking at the changes made in waveforms.py it all looks great to me, the code is much more cleaner than previous version!

(Sorry for sticking that much to the previously established code in my last merge, it probably ended up adding you some work!)

[rmodrak]

I was also checking that the different waveform normalization options continue to work-- will try to post the result later.