Add icon argument to getIdList() function.

[ZogopZ]

Extend getIdList() function by adding an icon argument.
By default, when running:
station.getIdList(project='icos', outfmt='map') the code adds a country flag to each station's location.

• Change default behavior to a folium's built-in default marker.
  station.getIdList(project='icos', outfmt='map')
• Add functionality for custom user defined icon.
  station.getIdList(project='icos', outfmt='map', icon='/home/.../small_sized_image.png')
• Keep the previous functionality when the user explicitly sets icon='flag'.
  station.getIdList(project='icos', outfmt='map', icon='flag')
• Update pylib's documentation to include these changes.