Land-sea mask is low resolution

[Sbozzolo]

The land sea mask we are currently using is only 64x64 points (in long-lat).

It is also a different land sea mask than ClimaLand.

[akshaysridhar]

A reasonable initial step towards improvements might be to use data provided in https://www.ncl.ucar.edu/Applications/Data/#cd under landsea.nc which contains a 360x180 (lon-lat) grid with 5 masking categories:

Global attributes
  title                = 1x1 Land-Sea Mask, 0=Ocean, 1=Land, 2=Lake, 3=Small Island, 4=Ice Shelf
  creation_date        = Thu May  6 10:51:06 MDT 2004
  source               = Based on Rand's Global Elevation and Depth Data, Modified Extensively
  source_web           = http://www.dss.ucar.edu/datasets/ds750.1/
  creator              = Adam Phillips, NCAR

We can collect information from even higher resolution datasets (following the Slack discussion) in a separate issue if necessary.

cc @sriharshakandala @kmdeck