Improve feature wrapper and fractal task

[jluethi]

I was anyway working on integrating the feature_wrapper function better into the Fractal task, so I also did a small refactor to address #59.

The new get_regionprops_measurements is simplified and calls 3 helper functions for the main measurements. In my tests, it still performs the same as before (except the C01.x_pos_weighted_pix etc. measurement being quite different, but that was an intentional bug fix, right?

Fractal testing is still ongoing, but this can already be merged now to avoid to many different branches in parallel.

[jluethi]

@enricotagliavini Let me know if you're happy with this cleanup approach :)

[enricotagliavini]

It's a good start, the logic is more clear and, in general, it's more readable. I might have some further ideas, let's also see what @nrepina thinks.

[nrepina]

Very nice, thank you @jluethi, and perfect timing for the refactor! I tested and also see no difference in output compared to feature_wrapper prior to refactor.

Indeed, the change in weighted z centroid was an intentional bug fix.

I will approve the merge and let's chat @enricotagliavini if we want to improve further.