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).