prioritizrutils: Utilities for prioritizr

#' @include internal.R
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#' Conservation problem constraints
#'
#' TODO
#'
#' \describe{
#'
#'   \item{\code{\link{add_connected_constraints}}}{
#' Add constraints to a conservation problem to ensure that all selected
#' planning units are spatially connected to each other.
#'   }
#'
#'   \item{\code{\link{add_corridor_constraints}}}{
#' It is important to maintain connectivity between reserves. However,
#' some areas are more difficult for species to traverse then other areas.
#' As a consequence, even though reserves may be connected, species may
#' not be able to move between reserves if the areas connecting them
#' are barriers to dispersal.
#'
#' This function adds constraints to ensure that corridors connect
#' reserves and that individuals from all species can utilise the
#' corridors. Friction \code{\link[raster]{Raster-class}} objects area
#' used to show each dificult areas are to traverse.
#' }
#'
#'   \item{\code{\link{add_locked_in_constraints}}}{
#' Add constraints to ensure that they are prioritized in the solution.
#' For example, it may be desirable to lock in planning units that are
#' inside existing protected areas so that the solution fills in the gaps in the
#' existing reserve network.
#' }
#'
#'   \item{\code{\link{add_locked_out_constraints}}}{
#' Add constraints to ensure that certain planning units are not prioritized
#' in the solution. For example, it may be useful to lock out planning
#' units that have been degraded and are not longer suitable for conserving
#' species.
#' }
#'
#'   \item{\code{\link{add_neighbor_constraints}}}{
#' Add constraints to a conservation problem to ensure that all selected
#' planning units have at least a certain number of neighbors.
#' }
#'
#' }
#'
#' @name constraints
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