meanCurvatureSq Functional Documentation

[Craftyleosteel]

When looking through both the manual and using the "?" command in morpho, we lack documentation for how this command is used. This should be added in both future updates in the manual as well as our current help function.

[Craftyleosteel]

Or alternatively, it should link to how to extract functionals. I think that this could help a lot with the user experience.