add imerg landsea mask artifact

[AlexisRenchon]

Checklist:

• I created a new folder imerg_landsea_mask
  • I added a README.md in that that folder that
    • describes the data and processing done to it
    • lists the sources of the raw data
    • lists the required citation, licenses
  • If applicable (e.g., for Creative Commons), I added a LICENSE file
  • I added the scripts that retrieve, process, and produce the artifact
  • I added the environment used for such scripts (typically, Project.toml
    and Manifest.toml)
  • I added the OutputArtifacts.toml file containing the information
    needed for package developers to add imerg_landsea_mask to their package
• I uploaded the artifact folder to the Caltech cluster (in
  /resnick/groups/esm/ClimaArtifacts/artifacts/imerg_landsea_mask)
• I added the relevant code to the Overides.toml on the Caltech Cluster
  (in /resnick/groups/esm/ClimaArtifacts/artifacts/Overrides.toml)
• I added a link to the main README.md to point to the new artifact

[kmdeck]

thank you!