incorrect handling of matrix-shaped states

[ghennequin]

This fails, despite what the README.md says about integration of matrices:

open Owl
open Owl_ode

let tspec = Types.(T1 { t0 = 0.; duration = 1.; dt = 1E-3 })
let solver = Owl_ode_sundials.cvode ~stiff:false ~relative_tol:0. ~abs_tol:1E-5
let n = 10
let batch_size = 3
let dx x _ = Mat.(neg x)
let x0 = Mat.gaussian batch_size n
let times, states = Ode.odeint solver dx x0 tspec ()

For reference, the README.md says:

We also support temporal integration of matrices. That is, cases in which the state x is a matrix of dimensions of dimensions NxM. By default, in the output, we flatten and stack the states vertically (i.e., ts has dimensions Tx1 and xs has dimensions TxNM.

Just logging this issue so I don't forget, will try to take a look later ─ probably a bug introduced in Dec 2019 with a commit called Arr integration.