logging/Exception instead of print, unique variabe names, return plots

[wingechr]

Hi @ceposadap. Could you merge these small changes into the code:

• changes in gas names in constants.py so that they are unique
• plot functions in make_plots.py return the plot
• replaced print statements with logging/raise Exception

here is a first draft of how the gst library will be used in the website: https://gist.github.com/wingechr/6c5a92d51c6fad817dc5e6c99c4343b9

Let's discuss it on a next meeting.