Update naming of constants in functions relating to Vapor Saturation Pressure

Hi @bavay, is this how you had imagined these functions to be modified?

In response to https://code.wsl.ch/snow-models/snowpack/-/merge_requests/8?diff_id=972&start_sha=1301145e60f0f7dc6dc0518f97601776a2735f12#note_7977