change `index` field to `target_size` field.

This commit moves 2e-integrals to a more CSR-like format, making
downstream concatenation much simpler.

generation/Integrals.jl

@@ -84,7 +84,7 @@ struct BiIntegral4c{T}  <: ArrayFields
     xyz :: Array{T}
     ijkl :: Array{Int16}
     Bijkl :: Array{Float32}
-    index :: Vector{Int16}
+    target_size :: Vector{Int32}
 end
 function BiIntegral4c(
     l :: Vector{Int64}, 
@@ -93,8 +93,8 @@ function BiIntegral4c(
     ijkl :: Array{Int16},
     Bijkl :: Array{Float32}
 ) where T<:Real 
-    index :: Array{Int16} = fill(0, size(Bijkl))
-    BiIntegral4c{T}(l, exp, xyz, ijkl, Bijkl, index)
+    target_size :: Array{Int32} = [length(Bijkl)]
+    BiIntegral4c{T}(l, exp, xyz, ijkl, Bijkl, target_size)
 end
 
 "Stack array fields, excluding constant fields"
@@ -117,11 +117,8 @@ end
 function stack(rows::Vector{BiIntegral4c}) :: BiIntegral4c
     out = map(rows[1]) do field
         k, fk = field
-        if k ∈ (:ijkl, :Bijkl)
+        if k ∈ (:ijkl, :Bijkl, :target_size)
             reduce(vcat, map(r -> getfield(r, k), rows))
-        elseif k == :index
-            idx(r::BiIntegral4c, i::Int) :: Vector{Int16} = i .+ r.index
-            reduce(vcat, map(idx, rows, 1:length(rows)))
         elseif k == :l
             rows[1].l
         else