Add watervapor landmask creation to utils.py #95

The watervapor data has diffrerent scope (lat till +-80 deg) and resolution (downsampled to 0.5 deg instead of 0.25 deg). This means we need to use a different land-sea mask.

Example script in commented out cell in watervapor NB

According to this discussion, we can downsample the LSM for SST with a factor of 2, and use >=0.5 as land (True) and <0.5 as water (False)