feat: flexEnz instead of flexProt + more detailed errors

[edkerk]

Main improvements in this PR:

• Features:
  • readDLKcatOutput error gives list of non-matching metabolites and reactions (solves #334)
  • mapRxnsToConv error if empty flux vector is used as input (solves #332)
• Refactor:
  • Use flexibilizeEnzConcs as function name for flexibilizing enzyme concentrations in the model.

I hereby confirm that I have:

• [x] Selected develop as a target branch (top left drop-down menu)

[github-actions[bot]]

This PR has been automatically tested with GH Actions. Here is the output of the tests:

 
Running geckoCoreFunctionTests
Done geckoCoreFunctionTests
__________

Note: In the case of multiple test runs, this post will be edited.