Reports the minimum or maximum daily feed value

Adds the option to record the minimum or maximum daily feed value, which resets to current feed value at midnight. The reset allows the function to operate with sub zero data (the mysql default is null which would otherwise create problems).
The feed is only updated whenever a 'new max' or 'new min' is encountered.
Thanks to Jerome for his help with this.
Paul
Adds min & max description for process_model.php update
Amendment to feed update test
Merge pull request #90 from Paul-Reed/patch-1

Reports the minimum or maximum daily feed value