Neater notebook representation of Model Objects

[achillesrasquinha]

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It would be nice to have a neater representation of the model objects - metabolites, reactions, genes, etc. For example,

This introduces a _repr_html_ within the DictList object. Each Object then declares a _DF_ATTRS internally that contains the needed column list for the data frame representation to be created. Adding a to_dataframe() function to DictList would solve this.

>>> model.reactions.to_dataframe()

I've currently already had this implemented at https://github.com/achillesrasquinha/cobrapy/tree/feats

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Additional context

[Midnighter]

Please make a new pull request for this. And don't add it to #1278. I like the idea, though. 👍🏼

Oh, I got confused because you kept the same title as for #1276