Improve motor demo

• comments for clarity
• commands now stack to increase and decrease velocity.
• increased maximum force-per-frame to each joint
• simplified nearcallback with lambda and removed some other redundancies

Hi @i-make-robots, thanks a lot for the PR!

As I understand, this is a purely cosmetic PR, correct?

Unfortunately, there is a problem, I usually do not accept cosmetic changes to ode4j. I know that the look of the code makes your toenails curl up, but there is a reason I keep it this way. ode4j is an ongoing port of ODE C++. Every bit of rearranging/adding/removing of the Java code makes it harder for all following iterations when I port the latest changes. It is not so bad if ODE has few changes, but they sometimes refactor their code, and then it becomes very hard because I have to compare Java and C++ side by side and for every difference I have to track back where it came from. Basically, I have been there many times and decided to only fix the most obvious (e.g. typos) things.

The fact that this is a demo (which is looked at by users) means that I may actually adopt some changes, but it is likely only a small subset of what you propose. I will have a look. Sorry about that.

I really do appreciate the effort though.