Compute Non-standard Statistics

Allow computation of statistics not directly implemented by the program
or the np.ma package.

Added two options:
add_stats = [‘stat1’, ‘stat2’]   # List of statistics to be computed
add_func=‘myfunc’    # Module where the functions that compute these
statistics are defined
Using a list as a default argument can lead to some strange behavior. As as rule, I'd rather have the default be `None` and later set it to an empty list. http://docs.python-guide.org/en/latest/writing/gotchas/#mutable-default-arguments