FRET should detect and reject duplicate requirement identifiers

[smithdtyler]

Kind2 throws errors if you try to do realizability analysis on a model with duplicate requirement IDs. It would be better to detect this in FRET.

[andreaskatis]

Hi @smithdtyler ,

I am currently working on improvements for #97 , that should also address this kind of errors. In the meantime each requirement must have a unique ID assigned to it for realizability checking.

Best,

Andreas