map: use OpenStreetMap instead of google

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there are bugs when using ggmap and openstreetmap

• https://stackoverflow.com/questions/23572996/ggmap-gives-error-when-using-open-street-map-as-source
• https://stackoverflow.com/questions/24965213/whats-going-on-with-get-map
• get_map cannot load OpenStreetMap dkahle/ggmap#172 : use stamen instead of osm : http://maps.stamen.com/#watercolor/12/37.7706/-122.3782

cf leaflet ? https://rstudio.github.io/leaflet/

if no way to debug osm with ggmap, maybe update the following pmap function code in common_fonctions.R:

  n = unique(d[, c("lat", "long", "location")])
  n$lat = as.numeric(as.character(n$lat))
  n$long = as.numeric(as.character(n$long))
  n = na.omit(n)
  center_location = c(mean(n$long), mean(n$lat))
  map = ggmap::get_map(location = center_location, source = "google", zoom = zoom)
  m = ggmap::ggmap(map, extent = "device")
  ggsave("tmp_map.png", m, width = 1, height = 1) # get a perfect square
  p = ggplot() # support for the map background
  p = p + coord_cartesian(xlim = range(m$data$lon), ylim = range(m$data$lat), expand = FALSE)
  img = png::readPNG("tmp_map.png")
  pmap = p + annotation_custom(grid::rasterGrob(img, width = unit(1,"npc"), height = unit(1,"npc")),
                               -Inf, Inf, -Inf, Inf) # change in the look of the map because of coordinates changes
  pmap = pmap + xlab("long") + ylab("lat")
  file.remove("tmp_map.png")

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With ggmap osm is deprecaed. See dkahle/ggmap#172

Instead of osm, use stamen : http://maps.stamen.com/#watercolor/12/37.7706/-122.3782

see ?get_stamenmap for more details

To settle attributions:
Map tiles by Stamen Design, under CC BY 3.0. Data by OpenStreetMap, under ODbL.

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done in v0.25