Blocked logical indexing

[mtfishman]

This introduces a blocked version of logical indexing. With this PR, if you index into array with an AbstractBlockVector{Bool}, it is interpreted as a logical index but also uses the blocking of the index to determine the block structure of the output array, for example:

julia> using BlockArrays

julia> a = randn(6, 6)
6×6 Matrix{Float64}:
 -0.0577235  -0.12942   -0.10982    1.01086    0.196898   0.896616
 -0.163481   -1.24784    1.01413    0.244657  -0.49961   -0.435926
  1.64239    -0.930051  -0.923835  -0.789608  -0.53113   -0.0502656
  0.0999888  -2.41073   -2.03078    0.019679  -0.857197   0.188939
 -0.698236   -0.218804  -1.36086    0.77242    0.1388     1.97166
  1.77482    -1.58258   -0.042804   1.30733    1.33004    0.930145

julia> mask = [true, true, false, false, true, false]
6-element Vector{Bool}:
 1
 1
 0
 0
 1
 0

julia> I = BlockedVector(mask, [3, 3])
2-blocked 6-element BlockedVector{Bool}:
 1
 1
 0
 ─
 0
 1
 0

julia> a[I, I]
2×2-blocked 3×3 BlockedMatrix{Float64}:
 -0.0577235  -0.12942   │   0.196898
 -0.163481   -1.24784   │  -0.49961 
 ───────────────────────┼───────────
 -0.698236   -0.218804  │   0.1388  

Without this PR, the output of a[I, I] would not have been blocked.

[codecov]

Codecov Report

Attention: Patch coverage is 73.33333% with 4 lines in your changes missing coverage. Please review.

Project coverage is 93.65%. Comparing base (e9c28d4) to head (a01c225).
Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
src/views.jl	71.42%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #475       +/-   ##
===========================================
+ Coverage    0.00%   93.65%   +93.65%     
===========================================
  Files          19       19               
  Lines        1689     1719       +30     
===========================================
+ Hits            0     1610     +1610     
+ Misses       1689      109     -1580     

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