#50 Address the handling of special taxids: -1, -2, ...

[Samuel KERRIEN]

As in the PSI-MI XML 2.5 paper:

Currently no taxonomy identifiers other than NCBI taxid are anticipated, apart from the use of -1 to indicate ‘in vitro’, -2 to indicate ‘chemical synthesis’, -3 to indicate ‘unknown’, -4 to indicate ‘in vivo’ and –5 to indicate ‘in silico’.