feat: complete MAR ids by filling blank fields

[haowang-bioinfo]

Main improvements in this PR:

• As agreed in feat: new sets of id for reactions and metabolites #174 , a complete list of rxnMAID is now obtained by:
  • replace underscore with R in MA ids
  • sequentially fill blank elements with unique MA-R ids

I hereby confirm that I have:

• [] Tested my code on my own computer for running the model
• Selected develop as a target branch

[mihai-sysbio]

I went ahead and refactored the id format in 86d7e2d . Feel free to discard the commit if there's been a mistake.