enable weighting

[ge-li]

Hi there,

I studied the structure of this package, although the manual says that adding weights to the pim model is not implemented, I found the essential parameter & vcov estimating scripts did indeed include the functionality of having weights.

I made changes to:

• "pim.R" - so it will create new pim class with weights slot
• "pim.class.R" - add weights slot to the class
• "pim.fit.R" - pass weights to the estimF()
• "Estimators.R" - add weights argument to estimator.nleqslv(), and pass weights to construct()

Please let me know if I can help more with this.

Best,
Li