Extend matmul with DimUnitRange axes

[sethaxen]

Extends matmul methods to construct an AbstractDimArray whenever one argument is an AbstractDimArray and the other has DimUnitRange axes. Fixes #1122

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.53%. Comparing base (2e5a812) to head (8b08997).
⚠️ Report is 6 commits behind head on breaking.

Additional details and impacted files

@@             Coverage Diff              @@
##           breaking    #1124      +/-   ##
============================================
+ Coverage     82.43%   82.53%   +0.09%     
============================================
  Files            57       57              
  Lines          5647     5673      +26     
============================================
+ Hits           4655     4682      +27     
+ Misses          992      991       -1     

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[sethaxen]

One coincidental behavior change is that before this PR dim(fill(1)) === nothing. After this PR dim(fill(1)) == (). Not certain if this has any bad side effects.

[tiemvanderdeure]

I guess there isn't really any answer to if fill(1) has dimensional axes

Maybe we can just add a dims(x::AbstractArray{<:Any, 0}) = nothing to not make this breaking

[rafaqz]

Yeah lets special case it thats slightly undexpected.

Actually, fixing this will create ambiguities everywhere so we have to define dims twice for AbstractDimArray. Lets see if it causes any real problems first.