Mismatch in with the `centroid_pos` keyword

[Jonathan Barnoud]

The centroid_pos keyword in <https://ffea.readthedocs.io/en/stable/keywordReference.html#blobBlock> is refered to as centroid, it is refered is centroid_pos in some of the test files.

Also, the unit is not specified in the documentation.

[Jonathan Barnoud]

The velocity keyword does not have a unit either in that documentation page.