make_transmission editing master file causes unnecessary issues on multiple nodes

[jfarr03]

When running make_transmission.py, its final step is to edit the master file to add the redshift grid used in LyaCoLoRe. When multiple nodes try to do this at once, it produces errors when it doesn't really need to (relating to io). A simple fix to only edit the master file from one node should suffice.

This doesn't cause any issues with the output of LyaCoLoRe, but would be good to fix to avoid confusion.