PR Summary

This PR prevents users from stumbling into#7122,#8779,#15501.
It does not technically fix them,FixedFormatter has a one-off bug with locators other thanFixedLocator. But setting fixed labels at unknown positions is anyway not a good idea. So even if the one-off bug was fixedFixedFormatter should only used withFixedLocator.

• This PR mostly documents thatFixedFormatter should not be used without aFixedLocator (along the hierarchy of tick label setter methods).
• Additionally it warns on the lowest levelset_major/minor_formatter() if aFixedFormatter is set without aFixedLocator. This may be debatable.
  One could set the formatter first and the locator afterwards, which would technically yield the correct result. However, I feel that setting the position first and the labels second is the natural way, and is actually done throughout the library.
  No alternative: Since Formatters don't know about LocatorsFixedFormatter itself cannot check at runtime and we have to resort to this somewhat clumsy approach.
  And I strongly want some feedback on this potential problem from running code, because nobody reads the docs 😄.
• One could consider captureing and re-emitting warnings on the higher levels with more appropriate messages, but I felt this was too much fuss.