Create sampling.pgls.R

This Functions perform simple sampling effort and power analysis of pgls linear regression.
This function onlye works with simple linear regression (Y ~ ab + e). Future implementation will deal with more complex models.
Function output is a data.frame with parameters estimates for each simulation. Further, 4 graphis to regression diagnostics:
1- Distribution of estimated betas
2- Estimated betas for each simulation 
3- Mean estimated betas per number of species
4- Power ~ % of species removed from original dataset