Improvement: Renamed forest functions to explicitly say leaf elements

[spenke91]

Closes #1604

Describe your changes here:

Based on the discussion we had regarding issue #1508 , this PR renames some forest-related functions: Their names now expicitly use "leaf_elements" whenever suitable.

Note the related but separate PR renaming some member variables: #1608

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[Davknapp]

does it close issue 1508? If so, can please mark it with one of the keywords and delete the automatically opened issue 1607

[spenke91]

does it close issue 1508? If so, can please mark it with one of the keywords and delete the automatically opened issue 1607

Thanks for pointing that out, I already fixed this typo. This PR does not close #1508, but only the subissue #1604. Once both #1604 and #1605 are closed, I will make sure to close #1508

NEWS.md is missing

[ole-alb]

Can you please add theses changes in the NEWS.md?