Activator backprop safety fix, totality test fix
    * Some of the totality tests not evaluating far enough
      to do anything -- fixed
    * Activator backprop was unsafe w.r.t preActivations size
        * Changed to unafeBackprop, added backprop
    * Changed Layer backprop to reflect the above
    * Fixed dimension mismatch in activator propogation
    * Changed around the dimension checking helpers to be less
      confusing
    * Added another Arbitrary wrapper to generate backprop-able layers
      and PData