Automatically create symlinks

[will-moore]

@joshmoore I'm wondering what's the best way to automate the creation of symlinks, for when we're dealing with a large number of Filesets.

It seems to me that one option is to add symlink creation into the omero mkngff sql command, so create the *_converted directory and links at the same time as generating the sql.
This could be optional behaviour under a --symlink flag, or we could add a separate symlink command that uses the same logic as def sql to get the prefix etc?

I was previously using a script to automate symlink creation at IDR/idr-utils#54