Fixed some bugs in DEMETER/mgPipe

[almut-heinken]

Fixed some bugs

• file reading in DEMETER that vcauses the file to read wrong with certain model names
• exchange reaction formulated wrong in DEMETER
• plot in mgPipe that crashes if all models have the same number of microbes
• diet models saved when not producing biomass
• fixed bug in creation of violin plots
• enabled entry of custom table and subset of taxa to create in createPanModels

I hereby confirm that I have:

• [x] Tested my code on my own machine
• [x] Followed the guidelines in the Contributing Guide
• [x] Selected develop as a target branch (top left drop-down menu)

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