Output prefix not forwarded for Bifrost #23

Hi,

When using the --out flag with the --write-graph flag, the resulting graph and colour files do not inherit the output prefix as expected but rather take it from the input reads/refs.txt file.
For example, when running the following command:

unitig-caller --call --refs refs.txt --out Arab_ME14 --pyseer --threads 8 --write-graph 

I'm getting the following files:

Arab_ME14.pyseer
refs.color.bfg
refs.gfa

The value provided to the --out flag should be forwarded to Bifrost (as the -o flag).