Warning message if array empty

Following fophillips comment on pull request #37, I've added warning
messages in the case that the array given to mean() or stddev() is
empty or contains only values that are NaN/infinite
Merge pull request #38 from roughhawkbit/master

Warning message if array empty
Changes to ContinuousVector required for eGUT epithelium. 1. Now
implements Comparable so that two CVs can be compared. 2. Implements the
required "compareTo" method.
Merge pull request #41 from kalden/master

Changes to ContinuousVector required for eGUT epithelium.