Phase overrides for different tmp_dirs

[racergoodwin]

I am plotting using a few different storage types, i.e. SSDs and raid 0 arrays. As you can imagine, they are of a different capacity and speed, so I have been using different scripts to maximise each as required and move the plots to different harvesters.
I like plotman so far, but have been losing out on quite a few plots per day by not being able to configure each tmp_dir individually, so I have made some changes, allowing overrides of all relevant variables and changed the config.yaml to reflect this.
I'm very new to github and source collaboration and have already made one pull request for something else, but don't know if I put it in the right place.
As this is a reasonable change, I'd like some guidance about how to suggest it gets merged, please?
Thanks.

[racergoodwin]

WIth good help and guidance, I have made a PR request, #758
Thank you.