Deprecate bzeros in favour of zeros(BandedMartrix,(n,m),(l,u)) a la GPUArrays.jl

[dlfivefifty]

Ideally this would be in Base, but GPUArrays.jl has started usage like

zeros(CLArray{Float32},(n,m))

for making a CLArray{Float32} that is n x m. Semantically I'm skeptical that this is the right choice (I prefer zero(CLArray{Float32},(n,m))), but I suppose we should follow the already established pattern and introduce:

zeros(BandedMatrix{Float64},(n,m),(l,u))

[dlfivefifty]

The plan is now to use FillArrays.jl, in which case it becomes:

BandedMatrix(Zeros{Float64}(n,m), (l,u))