[ENHANCEMENT] Define SparseArraysBase.isstored for block sparse arrays

[mtfishman]

Currently:

julia> using BlockSparseArrays: BlockSparseArray

julia> using SparseArraysBase: isstored

julia> a = BlockSparseArray{Float64}([2, 2], [2, 2])
2×2-blocked 4×4 BlockSparseMatrix{Float64, Matrix{Float64}, SparseMatrixDOK{Matrix{Float64}, BlockSparseArrays.GetUnstoredBlock{Tuple{BlockedOneTo{Int64, Vector{Int64}}, BlockedOneTo{Int64, Vector{Int64}}}}}, Tuple{BlockedOneTo{Int64, Vector{Int64}}, BlockedOneTo{Int64, Vector{Int64}}}}:
 .  .  │  .  .
 .  .  │  .  .
 ──────┼──────
 .  .  │  .  .
 .  .  │  .  .

julia> a[1, 2] = 12
12

julia> a
2×2-blocked 4×4 BlockSparseMatrix{Float64, Matrix{Float64}, SparseMatrixDOK{Matrix{Float64}, BlockSparseArrays.GetUnstoredBlock{Tuple{BlockedOneTo{Int64, Vector{Int64}}, BlockedOneTo{Int64, Vector{Int64}}}}}, Tuple{BlockedOneTo{Int64, Vector{Int64}}, BlockedOneTo{Int64, Vector{Int64}}}}:
 0.0  12.0  │  .  .
 0.0   0.0  │  .  .
 ───────────┼──────
  .     .   │  .  .
  .     .   │  .  .

julia> isstored(a, 1, 1) # Correct
true

julia> isstored(a, 1, 4) # Should be false
true

and:

julia> isstored(a, Block(1, 1)) # Not defined
ERROR: MethodError: no method matching isstored(::BlockSparseMatrix{Float64, Matrix{…}, SparseMatrixDOK{…}, Tuple{…}}, ::Block{2, Int64})
The function `isstored` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  isstored(::AbstractArray, ::Int64...)
   @ SparseArraysBase none:0
  isstored(::AbstractArray, ::CartesianIndex)
   @ SparseArraysBase ~/.julia/packages/SparseArraysBase/9oYN7/src/abstractsparsearrayinterface.jl:58
  isstored(::BlockSparseArrays.SparsePermutedDimsArrayBlocks{var"#s16", N, BlockType, Array} where {var"#s16", BlockType<:AbstractArray{var"#s16", N}, Array<:(PermutedDimsArray{var"#s16", N})}, ::Int64...) where N
   @ BlockSparseArrays ~/.julia/dev/BlockSparseArrays/src/blocksparsearrayinterface/blocksparsearrayinterface.jl:253
  ...

Stacktrace:
 [1] top-level scope
   @ REPL[18]:1
Some type information was truncated. Use `show(err)` to see complete types.

julia> isstored(a, Block(1), Block(1)) # Not defined
ERROR: MethodError: no method matching isstored(::BlockSparseMatrix{Float64, Matrix{…}, SparseMatrixDOK{…}, Tuple{…}}, ::Block{1, Int64}, ::Block{1, Int64})
The function `isstored` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  isstored(::AbstractArray, ::Int64...)
   @ SparseArraysBase none:0
  isstored(::AbstractArray, ::CartesianIndex)
   @ SparseArraysBase ~/.julia/packages/SparseArraysBase/9oYN7/src/abstractsparsearrayinterface.jl:58
  isstored(::BlockSparseArrays.SparsePermutedDimsArrayBlocks{var"#s16", N, BlockType, Array} where {var"#s16", BlockType<:AbstractArray{var"#s16", N}, Array<:(PermutedDimsArray{var"#s16", N})}, ::Int64...) where N
   @ BlockSparseArrays ~/.julia/dev/BlockSparseArrays/src/blocksparsearrayinterface/blocksparsearrayinterface.jl:253
  ...

Stacktrace:
 [1] top-level scope
   @ REPL[19]:1
Some type information was truncated. Use `show(err)` to see complete types.

julia> isstored(a, Block(1, 1)[1, 1]) # Not defined
ERROR: MethodError: no method matching isstored(::BlockSparseMatrix{Float64, Matrix{…}, SparseMatrixDOK{…}, Tuple{…}}, ::BlockIndex{2, Tuple{…}, Tuple{…}})
The function `isstored` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  isstored(::AbstractArray, ::Int64...)
   @ SparseArraysBase none:0
  isstored(::AbstractArray, ::CartesianIndex)
   @ SparseArraysBase ~/.julia/packages/SparseArraysBase/9oYN7/src/abstractsparsearrayinterface.jl:58
  isstored(::BlockSparseArrays.SparsePermutedDimsArrayBlocks{var"#s16", N, BlockType, Array} where {var"#s16", BlockType<:AbstractArray{var"#s16", N}, Array<:(PermutedDimsArray{var"#s16", N})}, ::Int64...) where N
   @ BlockSparseArrays ~/.julia/dev/BlockSparseArrays/src/blocksparsearrayinterface/blocksparsearrayinterface.jl:253
  ...

Stacktrace:
 [1] top-level scope
   @ REPL[21]:1
Some type information was truncated. Use `show(err)` to see complete types.

using:

julia> pkgversion(BlockSparseArrays)
v"0.2.5"

julia> pkgversion(SparseArraysBase)
v"0.2.6"

We could also consider defining isblockstored(a, 1, 1) == isstored(a, Block(1, 1)) though maybe the isstored interface is sufficient.