Draft: Improving the coupled phase changes code.

• Variables dth_i_up and dth_i_down now relate to half the element, which is just a matter of code consistency.
• When limiting the amount of melt given the amount of ICE and WATER, give priority to the phase changes in the upper part of element, and take the remainder to limit the lower part of the element. This avoids oscillations or blowups, where small amounts of total phase change can be created by huge phase changes with opposite signs in both the upper and lower part of the element.