Resolve "Review current state with ID21 data"

Closes #22 (closed)

Refactored the first stage of the pipeline: demo tasks, scan picking and fitting. I also started refactoring the regridding task.

I'm still not happy but I'll stop here for the MR.

I also added many tests for tasks and workflows.

Tutorial workflows are

1. Fit 1 scan 1 det:

2a. Fit 1 scan N det:

2b. Fit 1 scan N det (sum before fitting):

3. Fit N scan 1 det:

4. Fit N scan N det:

The demo tasks could return the scan URI's, making the pick scan tasks obsolete. But I wanted to design the tutorial workflows so that when you delete the data generating task, you have the workflow as you would us it in production. The arrows that come out of the data generating task reflect what the user needs to provide as input.