[OctDev] functions for GMM added to main/econometrics

[Michael C. <mic...@ua...>]

Hello,

I've added functions for generalized method of moments (GMM) estimation to the 
directory main/econometrics. The functions need the addition of help 
messages, but for now there is the script gmm_example.m, which shows how to 
use the set of functions.

The basic thing a user needs to do is define a function that returns moments 
for estimation using the framework

function m = my_moments(theta, data, otherargs)
< lines to calculate moments>
endfunction


In this framework:

* theta is a kX1 parameter vector

* data is an nXp matrix

* m is a nXg matrix of moment contributution

* otherargs is whatever else is needed to calculate the moments

The user must also provide a weight matrix. The example shows how to do this 
when the moments are uncorrelated.

Comments/questions welcome. 

Michael