Support syntax to construct zero, one, and fill BlockArrays

[dlfivefifty]

It would be nice to be able to construct Block arrays that are zeros.

A convention that could be copied is from GPUArrays.jl and override zeros, ones, and fill:

julia> zeros(CLArray{Float32}, 4, 4)
GPU: 4×4 Array{Float32,2}:
 0.0  0.0  0.0  0.0
 0.0  0.0  0.0  0.0
 0.0  0.0  0.0  0.0
 0.0  0.0  0.0  0.0

That is, support zeros(BlockArray{Float32}, 1:4, 1:4) to construct a 24 x 24 BlockMatrix.

I'm not that big a fan of this syntax as it is semantically the same as asking for a Matrix{CLArray{Float32}} (any comments @SimonDanisch ?).

An alternative would be to use zero instead of zeros, though then it would be inconsistent with GPUArrays.jl.

[dlfivefifty]

After discussions with SimonDansch, we decided it is better use a special type to do this, and so created FillArrays.jl I'm getting this working now:

BlockArray(Zeros(6,6), 1:3, 1:3)

[KristofferC]

I think using Zeros, Ones etc has the same problem that is currently causes constructors in Base to change? What if I want oneunit instead of one. Is there a need for a new type in that case as well? Using types. to chose what value something should be filled with seems to be lifting up the value domain to the type domain.

[dlfivefifty]

Then use Fill: BlockArray(Fill(oneunit(Dates.Day), 6,6), 1:3, 1:3)

[dlfivefifty]

Only 0 and 1 have special types as Zeros and Ones, with the latter being questionable. But Zeros is distinguished because of sparse matrices.

[dlfivefifty]

This almost works now that BlockArray(::AbstractArray, ...) works. The remaining issue will be resolved via #40.