ranghetti/sen2r: Version 1.3.5

Major changes

• GDAL is no longer a mandatory external dependency, being used only to smooth/bufferise cloud masks (and optionally to compute spectral indices, RGB images and thumbnails).
• "Graphical" packages needed to run the GUI (leaflet, leafpm, mapedit, shiny, shinyFiles, shinydashboard, shinyjs, shinyWidgets) are now suggested dependencies. In the case they are missing and the user tries to run the GUI, an error is returned with the command for installing them.

• Edit the GUI in order to disable selectors when dependencies required to run them are not available.
• Automatic tests were reorganised and improved after GDAL changes.

• Document sen2r installation over Ubuntu Focal.
• Update documentation according to GDAL changes.

• Fix Travis-lwgeom incompatibilities (#319)
• Fix #310 and #311