extract_standard: Extract the standard curve
In koncina/elisar: Elisa 4PL regression

Parse the id for the standard curve key and extract the concentration and associated O.D. values.

1 2	extract_standard(.data, concentration = "concentration", od = "od", std_key = "^STD", dec = ".", var_in = "value")

`.data`	dataframe containing at least the value and id columns (with O.D. values and sample identifiers).
`concentration`	name of the concentration column
`od`	name of the O.D. column
`std_key`	a character string specifying the common starting pattern of standard point ids (default = "^STD").
`dec`	a character string used as a decimal separator for the encoded standard concentration values.
`var_in`	a character string used for the OD values (default = "value")