ftest: calculate the F-statistic from two c2 values
Calculate the F-statistic and its probability given new and old values of and number of degrees of freedom (DOF).
Syntax: ftest chisq2 dof2 chisq1 dof1
The new and DOF, chisq2 and dof2, should come from adding an extra model component to (or thawing a frozen parameter of) the model which gave chisq1 and dof1.If the F-test probability is low then it is reasonable to add the extra model component. WARNING : it is not correct to use the F-test statistic to test for the presence of a line (see Protassov et al astro-ph/0201457).