Fix issues with shared config files

[xylar]

This merge fixes two main issues with shared config files:

1. The main one is adding an explicit attribute of steps to indicate whether they have a shared config (the alternative being that they use the config for the task they belong to). Previously, a step was assumed to have a shared config if it belonged to multiple tasks but that causes trouble when a conceptually shared step happens to belong to only one task (e.g. because other tasks may use it in the future).
2. The minor fix is that only the tasks and steps requested during setup have their config files linked. Previously, symlinks to the shared config were being created for all tasks and steps that used the config, even if they were not requested at setup. This created confusing directories for unrequested tasks that just had a symlink to the config file.

Checklist

• Testing comment in the PR documents testing used to verify the changes

[xylar]

Testing

I ran all ocean test cases except the 1 and 4 km baroclinic channel RPE tests (which are too expensive) on Chrysalis with Intel and OpenMPI. All tests were successful except manufactured solution, which failed the usual verification test.

I also tested #141 after rebasing onto this branch and it fixed the issues with the inception, wetting and drying tests that had previously been seen, see #141 (comment)

[xylar]

@cbegeman, can you just give this a quick review, probably just by inspection since I think my testing is sufficient?

[cbegeman]

@xylar Great, thanks! I had noticed the second issue so it's nice that you included it here. Approving by visual inspection.

[xylar]

Great, thanks for the rapid response, @cbegeman!