Add Windell Significance to Compound Indices

[sborstein]

“significance” of each food category, calculated as:
S = √(% occurrence)(%composition)

From:
Windell, J.T. 1971. Food analysis and rate of digestion. Pp. 215–226, In W.E. Ricker (Ed.).
Methods for Assessment of Fish Production in Fresh Waters. IBP Handbook No. 3,
Blackwell Scientific, Oxford, UK. 365 pp.