[WIP] convert Box to BoxND

[AlexanderSinn]

Summary

Similar to #3969 and #3988 but for Box.

Additional background

It should be checked that the changes to BoxIndexer do not affect the compiled GPU code. 

TODO: Remove AMREX_SPACEDIM and convenience functions.

Checklist

The proposed changes:

• [ ] fix a bug or incorrect behavior in AMReX
• [x] add new capabilities to AMReX
• [ ] changes answers in the test suite to more than roundoff level
• [ ] are likely to significantly affect the results of downstream AMReX users
• [ ] include documentation in the code and/or rst files, if appropriate