[ENHANCEMENT] Remove `BlockSparseArray{Float64}([2, 3], [2, 3])` constructor

[mtfishman]

Remove the BlockSparseArray{Float64}([2, 3], [2, 3]) constructor. Instead, either undef should be specified explicitly, i.e. BlockSparseArray{Float64}(undef, [2, 3], [2, 3]), or we should define a zeros constructor, i.e. blocksparsezeros(Float64, [2, 3], [2, 3]).

Also note that BlockArrays.jl has both BlockArray{Float64}(undef, [2, 2], [2, 2]) and BlockArray{Float64}(undef_blocks, [2, 2], [2, 2]) which distinguishes between the blocks being allocated with undefined elements vs. the blocks not being allocated yet. Maybe that distinction is relevant for certain block sparse types, or in constructors where you list the block locations (where undef vs. undef_blocks could distinguish between the blocks being allocated or not).

See @lkdvos's PR here: ITensor/SparseArraysBase.jl#33 doing the same thing for SparseArraysBase.jl.

[mtfishman]

This is a bit orthogonal, but something this reminds me of is that I wish Julia had a more systematic way of generically constructing special arrays, like zero or random arrays. It seems like the concept undef/UndefInitializer() could be generalized for that, for example:

blocksparsezeros(Float64, [2, 3], [2, 3]) == BlockSparseArray{Float64}(ZeroInitializer(), [2, 3], [2, 3])
blocksparserand(Float64, [2, 3], [2, 3], 0.4) == BlockSparseArray{Float64}(RandInitializer(; density=0.4), [2, 3], [2, 3])

(I think I saw this suggested somewhere but can't find it now, I'm sure this idea has come up before.)