Implement and document parallel transport on Grassmann.

[kellertuer]

I recently noticed (p. 171 AMS08), that parallel transport on Grassmann (Stiefel representation) has a closed form in the _direction variant. So here is an implementation. Will add tests the next few days.

🛣️

• [x] Test coverage
• [x] read documentation carefully that it renders correctly

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.58%. Comparing base (7785932) to head (d7057fb).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #731 +/- ## ======================================= Coverage 99.58% 99.58% ======================================= Files 114 114 Lines 11230 11244 +14 ======================================= + Hits 11183 11197 +14 Misses 47 47 ```

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