lcx.initialize: Starting parameters for an LCx model fit
In ahproctor/LC50: Estimate LC50 in the presence of additional stressors

Estimate starting parameters for an LCx model fit.

1	lcx.initialize(Y, conc, group, link = c("probit", "logit"), lethal)

`Y`	a two column matrix of the number of survivals and mortalities in each sample.
`conc`	a vector of toxin concentrations
`group`	a factor delineating treatment groups
`link`	the link function for survival fractions
`lethal`	the modelled level of lethality

This is the default method for computing the starting values used to initialize an lcx model.

Return a list of with components

`alpha`	the rate parameter for each treatment group
`gamma`	the probit of the control survival for each treatment group
`loglcx`	the log lcx for each treatment group