weights for propensity 'matching'

[dewittpe]

Implement and name five weighting methods within propensity

Method	Treated	Control
IPW, average causal effect in study population	1/pi	1/(1-pi)
Ave causal effect in treated	1	pi/(1-pi)
Ave causal effect in untreated	(1-pi)/pi	1
Ave causal effect in population for which sample is most informative	1-pi	pi
Ave causal effect in matching weight population	min(pi,(1-pi))/pi	min(pi,(1-pi))/(1-pi)

[tdbennett]

These weights are now implemented near the bottom of icppaper/propensity.R

[dewittpe]

These weights have been implemented in psweights.R.