Use "--config" option in PSyAD command

[TeranIvy]

In LFRic we use the psyclone command-line --config option to specify where our custom copy of psyclone.cfg is picked up from (LFRic Core in this case). If this option is not present, we default to the PSYCLONE_CONFIG environment variable set for a specific PSyclone release.

Now that we are porting LFRic to new systems, it became obvious that it would be good to have --config option for the psyad command as well, since it uses LFRic API but does not currently have a command-line mechanism for pointing to the custom PSyclone configuration file.

I would suggest prioritising this for 3.1 release (#2744). I am aware it is an API change.