feat: remove hard-coded params fitGAM

[edkerk]

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Some hard-coded parameters could be moved out of the fitGAM function, to make the code more versatile, and so no custom function edits would be required if a model would require non-default parameter values.

1. the scanned GAM range (GAM = 20:5:100) could be mentioned in `getModelParameters and set to these values by default?

2. the title and legend of the figure generated contain the name of the carbon source 2.1 fitGAM uses chemostatData.tsv, take the carbon source name from there? 2.2 alternatively, specify carbon source / growth medium name in getModelParameters?