Make subdirectories for symlinks for input files

[xylar]

This allows input files to be placed in a subdirectory of a step.

Checklist

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

closes #58

[xylar]

@darincomeau and @erinethomas, could you test out whether cherry-picking this commit fixes the issue we were having with the forcing directory?

git remote add xylar/polaris git@github.com:xylar/polaris.git
git fetch --all -p
# if you have uncommitted changes
git stash save
git cherry-pick cbc98eebeaaad6e89a36448dd70812e9e5ab95d8
# get back your uncommitted changes
git stash pop

[erinethomas]

yes this works! it now creates the forcing directory and links all the files correctly

[xylar]

Thanks @erinethomas and @darincomeau!