Plot int std spectrum and fit for data

I modified data.py so that it would generate plots of the internal standard
region of the data file if that calibration method is chose. This makes it
possible to ensure that the calibration region is correctly selected and
integrated.
Added png cleanup function

Added a function to clear out png files from the data and calibration folders
before processing the data. This helps to keep old pngs from building up in
the folders.
Cleanup data.py

I did some cleanup on data.py because the functions and processing code were
mixed in a confusing way. Also, I moved the code that checks a HDF table for
calibration information to the gmcs.py file so that it was not cluttering
things up.