Define dataids for PseudoBlockArrays

[jishnub]

This avoids various bugs that arise from aliasing.
An example on the current master:

julia> P = PseudoBlockArray(rand(4,4), [1,3], [1,3]);

julia> x1 = view(P, 2:4, 2:4);

julia> x2 = view(P, Block(2,2));

julia> x1 == x2
true

julia> x1copy = Array(x1);

julia> copyto!(x1, x2');

julia> x1 == x1copy'
false

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.52%. Comparing base (66e63c4) to head (efbdb29).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #364   +/-   ##
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  Coverage   94.52%   94.52%           
=======================================
  Files          16       16           
  Lines        1498     1499    +1     
=======================================
+ Hits         1416     1417    +1     
  Misses         82       82           

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