Adapted the 'metrics' subpackage docstring to the new package structure.

toarstats/metrics/__init__.py

-"""The toarstats package.
+"""The metrics subpackage.
 
-This package contains many different statistics and metrics that can be
-calculated over different time series.
+This subpackage contains many different statistics and metrics that can
+be calculated over different time series.
 
-This package exports the following function:
-calculate_statistics - public interface for the toarstats package
+This subpackage exports the following function:
+calculate_statistics - public interface for the metrics subpackage
 """
 
 from toarstats.metrics.interface import calculate_statistics