Define copy(::BlockIndexRange)

[mtfishman]

This defines copy(::BlockIndexRange), which fixes #404.

Before, copy(::BlockIndexRange) was outputting a Vector{<:BlockIndex}, which was leading to the behavior reported in #404. I think this definition of copy makes sense, since it is similar to the definition of copy for unit ranges (i.e. it just returns the unit range instead of collecting into a Vector).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.64%. Comparing base (4059149) to head (2db5671).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #460   +/-   ##
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  Coverage   93.64%   93.64%           
=======================================
  Files          19       19           
  Lines        1667     1668    +1     
=======================================
+ Hits         1561     1562    +1     
  Misses        106      106           

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