st: -gammafit-, -pgamma-, -qgamma- packages revised on SSC

[Nick Cox <njcoxstata@gmail.com>]

Thanks to Kit Baum as usual, these three packages have been revised on SSC.

The main change is to support an alternative parameterisation of the
gamma distribution directly.

In Stata 11, -gmm- has a worked example of fitting a two-parameter
gamma distribution directly using fake data from Greene's well-known
econometrics textbook. This example uses what these packages now
support via an -alternative- option.

Greene's text is described in http://www.stata.com/bookstore/ea.html

Various other small updates have been made to the help.

If interested, please install or update using -ssc- or -adoupdate-.

Nick

GAMMAFIT
 module to fit a two-parameter gamma distribution
 Authors: Nicholas J. Cox  Stephen P. Jenkins       Req: Stata version
8.0 (4.0 for gamma4)
 Revised: 2011-05-17

QGAMMA
 module to generate quantile-quantile plot for data vs fitted gamma distribution
 Authors: Nicholas J. Cox       Req: Stata version 8.0 (4.0 for pgamma4)
 Revised: 2011-05-17

PGAMMA
 module to generate probability plot for data vs fitted gamma distribution
 Authors: Nicholas J. Cox       Req: Stata version 8.0 (4.0 for pgamma4)
 Revised: 2011-05-17
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