Adding integration tests using a subset of indices in NGCD dataset (fixes #275)

Added integration/regression tests for climix with input from the NGCD dataset.

• Datasets with test configurations are specified in tests/integration/configuration.yml, and are used to parameterize test function in tests/integration/test_indices.py.
• Tests are run by using a custom command line option to pytest, pytest --test-config [config] or pytest -T [config], where config is one available in configurations.py, e.g.:

pytest -T ngcd_reg
pytest -T ngcd_org

• An optional argument can be used to run a specific test(s) within a configuration:

pytest -T ngcd_reg[cdd_yr]
pytest -T ngcd_reg[cdd_yr,dtr_yr,txn_yr]

• Indices tested are: cdd, cfd, csu, cwd, dtr, fd, id, prcptot, r1mm, r10mm, r20mm, rx1day, sdii, su, tnn, tnx, tr, txn, txx