Fixing fastGapFill, test succeeds

[tpfau]

Fixing a couple of issues in fastGapfill.

1. missing mat files are created when not existent, even if useFile is created (fails otherwise because they are missing)
2. Changing non-cell content access to content access, as otherwise reactions that have no sense (e.g. R_, nadh[c], _ex are created instead of R_nadh[c]_ex...

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• [X] Tested my code on my own machine
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