Change weight of coefficient in heat equation

[mdpiper]

This PR includes a slight change to the way temperature is diffused in the Heat model.

To show this, start with a 6x8 temperature array with an impulse of 100 at [3,4]:

[[ 0.   0.   0.   0.   0.   0.   0.   0. ]
 [ 0.   0.   0.   0.   0.   0.   0.   0. ]
 [ 0.   0.   0.   0.   0.   0.   0.   0. ]
 [ 0.   0.   0.   0. 100.   0.   0.   0. ]
 [ 0.   0.   0.   0.   0.   0.   0.   0. ]
 [ 0.   0.   0.   0.   0.   0.   0.   0. ]]

Advance the model by one time step.
In the original formulation, this gives:

[[ 0.   0.   0.   0.   0.   0.   0.   0. ]
 [ 0.   0.   0.   0.   0.   0.   0.   0. ]
 [ 0.   0.   0.   0.  25.   0.   0.   0. ]
 [ 0.   0.   0.  25.   0.  25.   0.   0. ]
 [ 0.   0.   0.   0.  25.   0.   0.   0. ]
 [ 0.   0.   0.   0.   0.   0.   0.   0. ]]

In the updated formulation, this gives:

[[ 0.   0.   0.   0.   0.   0.   0.   0. ]
 [ 0.   0.   0.   0.   0.   0.   0.   0. ]
 [ 0.   0.   0.   0.  12.5  0.   0.   0. ]
 [ 0.   0.   0.  12.5 50.  12.5  0.   0. ]
 [ 0.   0.   0.   0.  12.5  0.   0.   0. ]
 [ 0.   0.   0.   0.   0.   0.   0.   0. ]]

In each case, temperature is conserved, but in the second case, the diffusion is less rapid.