updated ts.schmidt.stability to handle NA's better

Skipping over rows in which there are NA's for all water temperature depths; running schmidt.stability() on rows with all NA's throws an error.
fixed missing parentheses
Merge pull request #43 from jzwart/patch-4

updated ts.schmidt.stability to handle NA's better
Merge branch 'master' of github.com:lawinslow/rLakeAnalyzer