Improve visualisations

[ekatef]

This PR introduces the following changes:

• Improve a scatterplot appearance
• Add a table with network.statistics() (closes #14)
• Fix a palette on scenarios and time-series plots, e.g. to avoid coal shown in green (closes #10)
• Improving content of spatial plots: p_nom removed, curtailment and used potential added
• Read model time resolution from the model itself (closes #10)
• Fix cursor for drop-downs changing is from default text mode to a pointer
• Customise theme
• Adjust table appearance to improve accessibility
• Adjust plots fonts to improve accessibility
• Add pop-up help suggestions to drop-downs:
  • [x] 1_Scenario_comparison
  • [x] 2_Spatial_configuration
  • [x] 3_System_operation
• Change name of the index tab

[ekatef]

changes look good to me

Thanks a lot for reviewing and testing it, @drifter089!