Do not require pyessv archive

[mishaschwartz]

The pyessv-archive is required to run any stac-populator CLI commands but should only be required to run the CMIP6 populator.

This PR allows the CLI to work as intended even if pyessv-archive is not installed. Instead of exiting with an error, it will show a warning and continue to provide all other functionality that does not require pyessv-archive to run.

Current behaviour:

$ stac-populator --help
Traceback (most recent call last):
...
...
STACpopulator.exceptions.ExtensionLoadError: /home/.../.esdoc/pyessv-archive directory does not exists

After this PR behaviour:

$ stac-populator --help
.../stac-populator/STACpopulator/cli.py:159: UserWarning: Could not load extension CMIP6_UofT because of error /home/mschwa/.esdoc/pyessv-archive directory does not exists
  warnings.warn(f"Could not load extension {populator_name} because of error {e}")
usage: stac-populator [-h] [--version] [--debug] {run} ...

This PR also updates the README.md to clarify the dependency on pyessv-archive and fixes some minor typos.

Finally, it changes the PYESSV_ARCHIVE_DIR variable in the Makefile to PYESSV_ARCHIVE_HOME which is the variable used by the pyessv project. This means that users who wish to install the archive in a non-default location only need to set one variable, not two.

[huard]

Looks good. FYI, trying to convince maintainers of CORDEX and CMIP CVs to host a jsonschema built on their CVs. Worst case, we can host those ourselves, and replace the validation using pyessv with a simple validate from jsonschema.