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
Once default is `None`, this will change to 
```
if add_stats is not None:
    if add_func is not None:
        ....
```