Helpful error message if the data path is wrong

[dorchard]

If the user provides a non-existent directory for --path (or a file) they get a pretty inscrutable runtime error, e.g.

> python -m nemo_spinup_forecast --ye True --start 20 --end 50 --comp 1 --steps 30 --path tests/data/nonexistent
...
Preparing ssh...
Using normal PCA
Traceback (most recent call last):
  ...
  File ".../forecast.py", line 185, in get_attributes
    self.files[-1], decode_times=False, chunks={"time": 200, "x": 120}
IndexError: list index out of range

This PR checks earlier on in the cli.py whether the path is valid then gives a message e.g.:

> python -m nemo_spinup_forecast --ye True --start 20 --end 50 --comp 1 --steps 30 --path tests/data/nonexistent
usage: __main__.py [-h] --path PATH [--ye YE] --start START --end END --steps STEPS [--comp COMP] [--ocean-terms OCEAN_TERMS]
                   [--techniques-config TECHNIQUES_CONFIG]
__main__.py: error: Data path does not exist: tests/data/nonexistent
  Please check the path and try again.