Reachability and controllability sets
Minor optimization

Don't compute reachability sets when they're not shown.
Reachability sets optimization

Simply only display the last tree circles: others are within them in
current configuration.
A small try on #21
Code style
Wrong language error check

Spaces in control statements mostly.
Introduce a little bit more order.
Update simulation based on elapsed time.
Right-click on reachability sets works.
Use blue instead of green for controllability set display.
[minor] Remove a non-sense comment
Dz location marker changed.

Fix #25, in a way: we changed the marker altogether, but now it's
centered over the exact spot.
Make landing spot draggable

Drag the circle to modify the desired landing spot.
Canopy handling is time based as well

The canopy mode is left unchanged based on keypresses because that
seemed more convenient.
[minor] Comment fix
Grammar
Handling the correct event.
Canopy and steady point are depicted using markers

fix #22
Simulation speed control from UI

fix #24
Add a canopy icon

Still don't know how to use it, i.e. rotate.
For #26
Strip unneeded <form> tags