Initialize measured snow height with nodata, such that measured snow height is "nodata" when not provided in the forcing data

Initialize measured snow height with nodata, such that it propagates through SNOWPACK as nodata when no measured HS is provided in the forcing data.

This is an attempt to fix a bug where the measured snow height is set to initial snow height from the *sno file, without further updates when measured snow height is not available in the forcing data. For example: when SNOW_EROSION is then set to TRUE, the snow drift routine will make a wrongful comparison of measured vs computed snow heights.

It also introduces several other safeguards against propagating nodata values in calculations involving measured snow height.