Time BC menu

• General notes on time-dependent boundary conditions
• "Preferences" menu item
• "Soil properties" menu item
• "Edit polygon courses" menu item
• "Delete all" menu item
• "Point potential or source" menu item
• "(Potential or source) In section" menu item
• "(Potential or source) Linear" menu item
• "Diffuse sources" menu item
• "Display all polygon courses" menu item
• "Special 1" menu item
• "Special 2" menu item
• "Special 3" menu item