Stack only takes scalars

[bstadie]

cgt.stack takes scalar inputs. There is currently no easy way to stack tensors.

The python backend actually uses np.array to do the stacking, so it will work as-is. The CPP backend would need some modification though to handle arbitrary stacking.

[joschu]

Stack can be implemented in terms of reshape and concatenate. That's seems to me like the most attractive approach, though it does add some extra reshape ops.
One reason to do it that way is that we'll probably want some optimizations specific to the Concatenate op. (E.g., we can avoid ever actually copying the data, assuming we're concatenating along axis=0.)

[robertnishihara]

This is addressed by #42.