Change type signature of _checkbuffers and _goodbuffers (#77)

These methods check the assumptions on colptr, rowval and nonzeros and thus should work for all AbstractSparseMatrixCSC. Changing the signature also removes the need for every subtype to implement it.

Fixes #9

src/sparsematrix.jl

@@ -79,8 +79,8 @@ end
 
 size(S::SparseMatrixCSC) = (getfield(S, :m), getfield(S, :n))
 
-_goodbuffers(S::SparseMatrixCSC) = _goodbuffers(size(S)..., getcolptr(S), getrowval(S), nonzeros(S))
-_checkbuffers(S::SparseMatrixCSC) = (@assert _goodbuffers(S); S)
+_goodbuffers(S::AbstractSparseMatrixCSC) = _goodbuffers(size(S)..., getcolptr(S), getrowval(S), nonzeros(S))
+_checkbuffers(S::AbstractSparseMatrixCSC) = (@assert _goodbuffers(S); S)
 _checkbuffers(S::Union{Adjoint, Transpose}) = (_checkbuffers(parent(S)); S)
 
 function _goodbuffers(m, n, colptr, rowval, nzval)