BlockArray construction from immutable array

[jishnub]

The following errors on master, but works after this PR:

julia> BlockArray(reshape(1:6, 2, 3), [1,1], [1,1,1])
2×3-blocked 2×3 BlockMatrix{Int64}:
 1  │  3  │  5
 ───┼─────┼───
 2  │  4  │  6

The container is allocated after computing the blocks, and not before it. This lets us obtain the type implicitly from the blocks.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.47%. Comparing base (ea33af1) to head (1cf0d79).
Report is 9 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #357      +/-   ##
==========================================
+ Coverage   92.66%   94.47%   +1.80%     
==========================================
  Files          16       16              
  Lines        1499     1501       +2     
==========================================
+ Hits         1389     1418      +29     
+ Misses        110       83      -27     

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