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
2002, ApJ 571, 545). WARNING: this command can only be used if the extra model
component is additive, this does not give the correct result if the component
is multiplicative (see Orlandini et al. 2012, ApJ 748, 86).