compute_economic_value: Compute Economic Values Based On Price Changes
In pvrqualitasag/MeatValueIndex:

In this function, the economic value is computed as the change in expected average price over a population for a given change in the population mean. The computation of the economic value is simplified by the assumption that the costs can be kept constant for the different levels of the population mean. Futhermore, we assume that the change in population mean does not change the amount of product produced. The result is standardized and expressed per unit of the trait.

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compute_economic_value(pn_mean, pn_sd, pvec_class_freq = NULL,
  pvec_threshold = NULL, pvec_price, pn_delta_mean, pn_gen_sd = NULL,
  pb_verbose = FALSE)

`pn_mean`	empirical population mean
`pn_sd`	empirical population standard deviation
`pvec_class_freq`	vector of empirical trait frequencies
`pvec_threshold`	vector of thresholds given by payment system
`pvec_price`	vector of prices given by payment system
`pn_delta_mean`	small change of population mean
`pn_gen_sd`	genetic standard deviation of trait
`pb_verbose`	level of output verbosity