Moved the calculation of metrics to its own subpackage titled 'metrics'.

toarstats/metrics/__init__.py

toarstats/metrics/constants.py

toarstats/metrics/defaults.py

toarstats/metrics/input_checks.py

toarstats/metrics/interface.py

toarstats/metrics/ozone_metrics.py

toarstats/metrics/solar_position.py

toarstats/metrics/stats.py

toarstats/metrics/stats_utils.py

toarstats/metrics/tests/.gitignore

toarstats/metrics/tests/__init__.py

...metrics/tests/create_sample_data_and_reference_results.py

toarstats/metrics/tests/test_toarstats.py