Add some basic plotting functions

[sandra-neubert]

Create some functions that allow to visualise:

• the generated data
• a prioritizr output

We will do this by creating separate functions:

• [x] elsar_plot_static_raster_c() for continuous data (including options for ggplot and tmap)
• [x] elsar_plot_static_raster_d() for discrete data (including options for ggplot and tmap)
• [ ] elsar_plot_static_vector_c() for continuous data (including options for ggplot and tmap)
• [ ] elsar_plot_static_vector_d() for discrete data (including options for ggplot and tmap)
• [ ] wrapper around all above: elsar_plot_static() with options: data_type = c or d and spatial_data = raster or vector
• [ ] elsar_plot_interactive() for the same options as above but using only leaflet (maybe add tmap option)
• [x] an addition to include contours or polygons of current PAs, boundaries or other if wanted
• [x] an addition to add logos etc. if wanted

@scabecks thoughts?

[sandra-neubert]

Note: I will not focus on tmap options for now: use ggplot for static plots and leaflet for interactive plots (after chatting about this with @scabecks and hearing abozt some previous tmap issues)