Handle 3D pencil tomo

[CJ-Wright]

Currently our pencil tomo is 2D (x, th). We'll need 3D tomo though, which is a stack of 2D tomo reconstructions. This means we need some logic which accumulates over Z but updates over all other variables.

def acc(old, new):
    oldz, oldtomo = old
    newz, newtomo = new
    if oldz == newz:
        oldtomo[..., -1] = newtomo
    else:
        oldtomo = np.concatinate(oldtomo, newtomo)
    return newz, oldtomo

We may need some initialization logic to make sure it looks 3D. I'm not sure how mayavi is going to act when it gets an (n, n, 1) array but we can figure it out.
We will also need to modify the metadata to tell us what the z motor is.