Add new metrics: standard deviation and median of gradient

[PinkShnack]

Kyoo was working on a std metric and it seems to work well for certain samples (beads). I have implemented these in a branch of nrefocus on my fork. I could create a merge request if it is something that would enhance nrefous.

To do:

• Add std and med metrics
• Test new metrics (and check if old ones need more tests)
• Add an example showing a comparison for the example cell image.

[paulmueller]

Definitely 👍 💪