Typo: "residue" has to be "residual"

Correct typo in:

• function names
• documentation

changed milestone to %2.1.0

added accepted category: documentation labels

assigned to @luenensc

added cleanup/refactoring label

mentioned in merge request !422 (merged)

mentioned in commit f3784668

closed with merge request !422 (merged)

mentioned in commit 63b55c6d

mentioned in commit 684dc8a0

mentioned in commit 8f7a90e4