[Feature request] fastg to gfa converter

[Vini2]

Is your feature request related to a problem? Please describe.
Currently, phables takes assembly graph files only in gfa format.

Describe the solution you'd like
Add a converting step if fastg files are provided to convert to gfa format.

[pdec]

The fastg2gfa referred in the docs for converting MegaHIT's assembly graph is a bit confusing and points to a deprecated repo.
I tried the gfatools and it seems to properly convert the .fastg into .gfa with gfatools view graph.fastg > graph.gfa