do not discard empty blocks in broadcast

[abraunst]

This avoids discarding empty blocks in broadcast of BlockArrays. Fixes #512.

The problem is described in the issue. Simple bug this PR fixes:

x = mortar([1:3, 1:0, 2:4])
y = ones(6)
z = y .+ x  # error

EDIT: In addition, the last commit fixes

x = mortar([1:1, 1:0, 1:2])
y = mortar([1:1, 1:0, 1:0, 1:2])
z = y .+ x

The changes are:

• fix sortedunion so that it doesn't elide empty blocks, move all sortedunion methods to blockaxis.jl
• fix SubBlockIterator iteration to cope with empty blocks
• add several tests with empty blocks

Let me know if you need clarifications

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.83%. Comparing base (9a2a9c6) to head (ef6f297).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #518   +/-   ##
=======================================
  Coverage   94.82%   94.83%           
=======================================
  Files          20       20           
  Lines        1914     1917    +3     
=======================================
+ Hits         1815     1818    +3     
  Misses         99       99           

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[abraunst]

I just realized that there's another sortedunion for strided vectors (that also erases empty blocks). Let me come up with a fix.

EDIT: done

[abraunst]

So I fixed the problem with InfiniteArrays from the tests. I'm afraid that combine_blockaxes was (and still is) a bit fragile...