Fix: missing gravity term in set_trajectory() function

[marvin-oh]

Description

Fixes a bug in SimpleTrajectoryPlanner.set_trajectory() function.
There was a missing term for gravity in the calculation of the y-axis position of the parabolic equation.

Fixes #8

Type of change

• Bug fix
• New feature
• Breaking change

[tellarin]

@marvin-oh Did you need to make any changes to successfully run RandomAgent or PigShooter?

[marvin-oh]

I found that both RandomAgent and PigShooter, which use SimpleTrajectoryPlanner.set_trajectory(), are not hitting the target location exactly. After applying the revised code, they reach the intended point precisely.

But, I recommend verifying it in your own environment since this work was done quite long ago.