Return covariances from utils/linfit_plot.pro

Add ability to return covariances from utils/linfit_plot.pro.  This requires
a pirated version of LADFIT from the IDL library (utils/ladfit2.pro) that
computes the covariance on the initial least-squares fit for return.  Minor
coding neuroses addressed in utils/linfit2.pro.

	new file:   utils/ladfit2.pro
	modified:   utils/linfit2.pro
	modified:   utils/linfit_plot.pro