Add hatching functionality to map functions

The current map plotting functions don't have the hatching feature. Here is a script to add hatching over a map from @vtorralba (not sure if it is the most updated version; if not, please help us update it, thanks!) It should work with VizEquiMap() but not other projections.

FYI @erifarov @vagudets

hatching<-function(lats, lons, mask, dens = 10, ang = 45, col_line = '#252525', 
                     lwd_size = 0.5, crosshatching =FALSE) {

  # Function to add a mask over the maps 
  # July 2016. Veronica Torralba
  
  # Arguments: 
  # ~~~~~~~~
  # lats:	Array of latitudes.
  # lons:	Array of longitudes.
  # mask: matrix with dimensions (longitud x latitud)
  # dens: the density of shading lines, in lines per inch. The default value of NULL means that no shading lines are drawn. 
  #        A zero value of density means no shading nor filling whereas negative values and NA suppress shading (and so allow color filling).
  # ang: the slope of shading lines, given as an angle in degrees (counter-clockwise).
  # col_line: colour of the lines
  # lwd_size: The line width, a positive number, defaulting to 1. The interpretation is device-specific, and some devices 
  #           do not implement line widths less than one. 
  # crosshatching: T/F if crosshatching instead of diagonal lines. 
  
  if (length(dim(mask))!= 2){
    stop('The mask matrix should have longitude x latitude dimensions')
  }
  
  if (dim(mask)[1]!=length(lons)| dim(mask)[2]!=length(lats)){
    stop('The mask matrix should have longitude x latitude dimensions')
  }
  
 # Function from www.menugget.blogspot.com
 # ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
  matrix.poly <- function(x, y, z = mat, n = NULL) {
    if (missing(z))
      stop("Must define matrix 'z'")
    if (missing(n))
      stop("Must define at least 1 grid location 'n'")
    if (missing(x))
      x <- seq(0,1,,dim(z)[1])
    if (missing(y))
      y <- seq(0,1,,dim(z)[2])
    poly <- vector(mode = "list", length(n))
    for (i in seq(n)) {
      ROW <- ((n[i] - 1) %% dim(z)[1]) + 1
      COL <- ((n[i] - 1) %/% dim(z)[1]) + 1
      
      dist.left <- (x[ROW] - x[ROW - 1]) / 2
      dist.right <- (x[ROW + 1] - x[ROW]) / 2
      if (ROW == 1)
        dist.left <- dist.right
      if (ROW == dim(z)[1])
        dist.right <- dist.left
      
      dist.down <- (y[COL] - y[COL - 1]) / 2
      dist.up <- (y[COL + 1] - y[COL]) / 2
      if (COL == 1)
        dist.down <- dist.up
      if (COL == dim(z)[2])
        dist.up <- dist.down
      
      xs <-
        c(x[ROW] - dist.left, x[ROW] - dist.left, x[ROW] + dist.right, x[ROW] +
            dist.right)
      ys <-
        c(y[COL] - dist.down, y[COL] + dist.up, y[COL] + dist.up, y[COL] - dist.down)
      poly[[i]] <- data.frame(x = xs, y = ys)
    }
    return(poly)
  }
  
  M <- list(x = lons, y = lats, z = mask)
  incl <- which(M$z ==TRUE)
  polys <- matrix.poly(M$x, M$y, z = M$z, n = incl)
  for (i in seq(polys)) {
    polygon( polys[[i]], density = dens, angle = ang, border = NA, lwd = lwd_size,col=col_line)
    if (crosshatching) {
      polygon(polys[[i]],  density = dens, angle = -ang, border = NA, lwd = lwd_size, col=col_line)
    }
  }
}