Add functions to import the datasets

[stefsmeets]

In this PR I set up the basic setup.py, and add some functionality to load the timeseries data.

from esmvaltool_sample_data import load_timeseries_data
cubelist = load_timeseries_data()

Needed for ESMValGroup/ESMValCore#856

[stefsmeets]

I have some issues with the dataset:
CMIP6.CMIP.FIO-QLNM.FIO-ESM-2-0.historical.r1i1p1f1.Amon.ta.gn.v20191204
which raises
ValueError: Cube 'air_temperature' must contain a single 1D y coordinate.

[stefsmeets]

Two more datasets give warnings:

esmvaltool_sample_data/data/timeseries/CMIP6/CMIP/NCAR/CESM2-WACCM-FV2/historical/r1i1p1f1/Amon/ta/gn/v20191120
/home/stef/miniconda3/envs/esmvaltool/lib/python3.8/site-packages/iris/fileformats/_pyke_rules/compiled_krb/fc_rules_cf_fc.py:2290: UserWarning: Gracefully filling 'lat' dimension coordinate masked points
  warnings.warn(msg.format(str(cf_coord_var.cf_name)))
/home/stef/miniconda3/envs/esmvaltool/lib/python3.8/site-packages/iris/fileformats/_pyke_rules/compiled_krb/fc_rules_cf_fc.py:2290: UserWarning: Gracefully filling 'lat' dimension coordinate masked points
  warnings.warn(msg.format(str(cf_coord_var.cf_name)))

esmvaltool_sample_data/data/timeseries/CMIP6/CMIP/NCAR/CESM2-FV2/historical/r1i1p1f1/Amon/ta/gn/v20191120
/home/stef/miniconda3/envs/esmvaltool/lib/python3.8/site-packages/iris/fileformats/_pyke_rules/compiled_krb/fc_rules_cf_fc.py:2290: UserWarning: Gracefully filling 'lat' dimension coordinate masked points
  warnings.warn(msg.format(str(cf_coord_var.cf_name)))
/home/stef/miniconda3/envs/esmvaltool/lib/python3.8/site-packages/iris/fileformats/_pyke_rules/compiled_krb/fc_rules_cf_fc.py:2290: UserWarning: Gracefully filling 'lat' dimension coordinate masked points
  warnings.warn(msg.format(str(cf_coord_var.cf_name)))

[bouweandela]

Two more datasets give warnings:

If they don't work, just delete the files and add them to the ignore list in datasets.yml so they will not be downloaded again.

[stefsmeets]

This is ready to be merged for all I'm concerned. @nielsdrost could you have a look?
I will address problems with the setup.py in a separate PR (metadata etc.).

[nielsdrost]

Looks good to me!

I also tried it out. Package seems to work just fine :-)