Refactor "move excess soil water upward" code to shared function

We commonly need to check that soil layers are oversaturated and, if so, move the excess soil water up by one soil layer (or runoff if the top layer is oversaturated). This happens in [least](https://github.com/ESCOMP/CTSM/blob/19d9b8a35cb433486c5e8d914a11e1c45f4e92c3/src/biogeophys/SoilHydrologyMod.F90#L1352-L1362) [three](https://github.com/ESCOMP/CTSM/blob/19d9b8a35cb433486c5e8d914a11e1c45f4e92c3/src/biogeophys/SoilHydrologyMod.F90#L2511-L2521) [places](https://github.com/ESCOMP/CTSM/blob/19d9b8a35cb433486c5e8d914a11e1c45f4e92c3/src/biogeophys/SoilWaterMovementMod.F90#L1404-L1409). This should be modularized to avoid repeated code.