sparsify is very slow

[alefcastelli]

consider changing the "sparsify" function (invoked when argument "tol" of PTDF is set) in the following way:

-get the ptdf.data -data = sparse(ptdf.data) -data.nzval[abs.(data.nzval) <= tol] .= 0 -SparseArrays.dropzeros!(data)