add constructors for uninitialized arrays

[bjarthur]

title says it all. tests pass locally on julia 1.3:

julia> CircularArray{Float32}(undef, 2,3)
2×3 CircularArray(::Matrix{Float32}):
 NaN  4.7198      3.6f-43
 NaN  4.5642f-41  0.0

[codecov]

Codecov Report

Attention: Patch coverage is 56.52174% with 10 lines in your changes are missing coverage. Please review.

Project coverage is 86.30%. Comparing base (ec93323) to head (b6f58ba).

Files	Patch %	Lines
src/CircularArrays.jl	56.52%	10 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##            master      #32       +/-   ##
============================================
- Coverage   100.00%   86.30%   -13.70%     
============================================
  Files            1        1               
  Lines           50       73       +23     
============================================
+ Hits            50       63       +13     
- Misses           0       10       +10     

Flag	Coverage Δ
unittests	86.30% <56.52%> (-13.70%)	⬇️

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[Vexatos]

Genuinely not sure about this one. CircularArrays is a generic package that supports all types of AbstractArray. I am not too fond of the idea of adding utility methods which are hardcoded to the standard Array type. And typing CircularArray(Array{Float32}(undef, 2, 3)) is not that much more work.

[Vexatos]

After thinking about it more, I still maintain that this doesn't belong in a generic package. The "verbose" way to create such an array mentioned above isn't that much more work, and this is functionality that could easily be added to a separate, higher-level package.