Add data type and commands selection

This pull request adds the necessary code to select the different available
data types for a given study and once those are selected it lets you select the
commands you want to run on the data.

Note that after you click Start Processing it's going to give you an error,
which is fine, we just want to have these two steps reviewed and in the repo.

@Squirrelo and I worked on this PR so neither of us should review it.
minor: this comment is off - you did that when you created datatypes variable as set. Here you're getting the sorted list...