Changes since version 0-1.2
* Models with only one explanatory variable resulted in a bug. This has been fixed.
* Estimation methods are now available with four functions : plm, pvcm, pggls and pgmm instead of one (plm) in the previous version.
* pvcm is a new function which estimate variable coefficients models. The "nopool" model is now part of it.
* pggls is a new function which enables the estimation of general FGLS.
* pgmm is a new function for general method of moments estimator.
* for all estimation functions, the four first arguments are now formula,data,effect,model
* robus inference is now provided by the pvcovHC function.
