index_calc: Function to estimate annual index
In rooperc4/MEHRSI: Habitat Model Estimates of Survey Abundances

This function takes the best fitting model and resamples the data refitting and recomputing the indices to come up with a bootstrapped error estimate for the annual index value.

1 2	index_calc(parameter_ests, best_model, best_parameters, variables, year, minob, boot_reps)

`parameter_ests`	Bootstrap estimates of parameters
`best_model`	for each variable, a form for the model equation (1, 2 or 3)
`best_parameters`	fitted values for each of the parameters
`variables`	Set of habitat variables to use in the modeling
`year`	years of the survey
`minob`	1/2 of the minimum CPUE observation for backtransformation of the data
`ob_CPUE`	Observed CPUE at the transect
`pred_pa`	Predicted presence or absence at the transect