Keyboard Control: Improved

**The controls are now incremental.**

Changelog:
- BASE value of force set to 2 for fine adjustment.
- The `keyboard-control` node subscribes to the force data and makes changes to this data, rather than setting an absolute value.
- Two bottoms for increasing and reducing the depth at which the bot is. (`top` and `bottom`)
- An emergency `STOP` button which will kill all the thrusters. And essentially bring the vehicle to a stop just after it loses it's inertia.

Todo:
- Show the values of the force on the GUI. (This can help in changing the values of the force.)
- Figure out a way to give impulse to the vehicle so as to set the heading of the bot. (i.e. Rather than giving a force continuosly, which will result in an acceleration, we will switch off the thruster after some time so as to  give it a heading in a particular direction and some velocity.)