Advanced F calculator spreadsheet added

Interest in a more accurate method than the the trapezium or trapezoid method for determining the area, F value,  under the lethal rate time plot has continued. Simpson's rules provide a more accurate way of calculating the area under the curve and an Excel spreadsheet has been programmed to use both rules to determine the F value of thermal processes and  is now  available for download.  Fahrenheit and Celsius versions have been provided as has D, Z, and reference temperature data for major food spoilage organisms. Additionally a z-value converter to change temperature from Celsius to Fahrenheit and vice versa is available.