Fix nested broadcasting of BlockedArray

[mtfishman]

Fixes #295. That issue seemed to be caused by code in the generic block broadcasting logic such as

BlockArrays.jl/src/blockbroadcast.jl

Lines 121 to 129 in 90ddfa9

	subblocks(::Any, bs::Tuple{Vararg{AbstractUnitRange{<:Integer}}}, dim::Integer) =
	(nothing for _ in blockaxes(bs[dim], 1))
	function subblocks(arr::AbstractArray, bs::Tuple{Vararg{AbstractUnitRange{<:Integer}}}, dim::Integer)
	return SubBlockIterator(arr, bs, dim)
	end
	@inline _bview(arg, ::Vararg) = arg
	@inline _bview(A::AbstractArray, I...) = view(A, I...)

assuming the broadcasting expression is already flat. This PR adds a call to Broadcast.flatten to explicitly flatten the broadcast expression in the generic block broadcast code.

It appears that at some point BlockedStyle broadcasting expressions were being flattened in this Broadcast.instantiate definition:

BlockArrays.jl/src/blockbroadcast.jl

Lines 199 to 202 in 90ddfa9

	@inline function Broadcast.instantiate(bc::Broadcasted{Style}) where {Style <:BlockStyle}
	bcf = Broadcast.instantiate(Broadcast.flatten(Broadcasted{Nothing}(bc.f, bc.args, bc.axes)))
	return Broadcasted{Style}(bcf.f, bcf.args, bcf.axes)
	end

but that was changed in #193. Reverting that change broke the tests introduced in #193 so I guess it is not safe to do that.

I have to admit I'm not sure if this is the best place to call Broadcast.flatten, a lot of this broadcasting code is hard to follow. The place I put it in this PR fixes #295, doesn't break other tests, and is hopefully least likely to interfere with custom implementations of block broadcasting in downstream packages.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.04%. Comparing base (a4a0b93) to head (25ae39b).
⚠️ Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #485      +/-   ##
==========================================
+ Coverage   93.81%   94.04%   +0.23%     
==========================================
  Files          19       19              
  Lines        1761     1764       +3     
==========================================
+ Hits         1652     1659       +7     
+ Misses        109      105       -4     

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