identifyFirstDay: Identify the type of the first day of a study
In rte-antares-rpackage/antaresFlowbased: Run FlowBased Simulations on Antares

This function identifies the nature of the first day of a study, by reading the input load data of the study designated by opts (calculation of the mean profile of each day to identify the weekend). The type of first day is represented by a number : 1 = Monday, 2 = Tuesday, ..., 7 = Sunday.

1 2	identifyFirstDay(opts, firstArea = "fr", secondArea = c("fr", "de", "be", "nl"))

`opts`	`list` of simulation parameters returned by the function setSimulationPath : path of the Antares study. By default, the value is indicated by `antaresRead::simOptions()`.
`firstArea`	`character` name of the area(s) to use to calculate the type of first day. By default, France (`"fr"`) is chosen.
`secondArea`	`character` name of additional area(s) to use to check the first day. By default, the value is `c("fr", "de", "be", "nl")` (France, Germany, Belgium, the Netherlands). It can be `NULL` but the first day will be determined with less certainty.

## Not run: 
# Identification of the first day of an Antares study where areas fr, be, de, nl exist
opts <- setSimulationPath("pathToAntaresStudy")
identifyFirstDay(opts)

## End(Not run)