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
add_stats & add_func

Missed description of add_func in previous
default was failing due to add_stats

Corrected
Updated and corrected

Uses dict instead for add_stats
Corrected test
Merge pull request #48 from ozak/add-stats

Compute Non-standard Statistics by passing additional stats functions
Merge branch 'add-stats'
Merge remote-tracking branch 'upstream/master'