Given a density function specified by the root of the density function
name, returns the integral over a specified range, usually the whole
real line. Used for checking that the integral over the whole real line
is 1.
The name of the density function to be integrated must be supplied as
the characters of the root for that density (e.g. norm,
gamma). The density function specified is integrated
numerically over the range specified via a call to
integrate. The parameters of the distribution can be
specified, otherwise the default parameters will be used.