Add temperature decay to equipment/mash

[Philip]

Assuming the outside air temp is constant, the mash temp should decay exponentially according to the formula:
T(t) = [T(0)-T_out] * exp(-k*t) + T_out,
where T(0) is the initial temperature, and T_out is the outside temperature. The constant "k" is dependent on the insulation and surface area of the mash tun. If we can make a wizard, guiding the user to put a specified amount of hot water in the tun and measure the temperature accurately at several time points, we can determine the value of "k" for the mash tun.
Then, we can accurately set the final temperature of each mash step (which we are currently not using).