Change work directory basename from 'step' to 'run'

Also ensure that 'dakota_cleanup' script finds these directories.
Add calculate() method and response_statistic member

calculate() was factored out of teardown(). The default
response_statistic is 'mean'.
Default statistic should be 'mean', not 'sum'
Add more unit tests for HydroTrend class
Fix mistake: 'median', not 'max' function
Update response variables, response levels, and comments
Pass output_files and response_statistic to HydroTrend()
Nothing to plot in this experiment
Update experiment description