Improve n-dimensional slicing of BlockIndexRange

[mtfishman]

Fixes #355. This is also fixed as a part of #462 but this PR is narrower in scope so could be merged more quickly.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.68%. Comparing base (f6c5555) to head (a38d45e).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #463   +/-   ##
=======================================
  Coverage   93.67%   93.68%           
=======================================
  Files          19       19           
  Lines        1677     1678    +1     
=======================================
+ Hits         1571     1572    +1     
  Misses        106      106           

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[dlfivefifty]

@mtfishman I think you have write access? Feel free to merge small changes like this, if downstream tests pass and you are confident it isn't going to cause problems.

[mtfishman]

@mtfishman I think you have write access? Feel free to merge small changes like this, if downstream tests pass and you are confident it isn't going to cause problems.

Yes, I have write access, will do.