`SimpleScoredSamplingPlanner`- try fallback generators even if valid trajectories were found (option)

[rayvburn]

Why this MR was created?

Currently It is impossible to use multiple trajectory generators that are treated equally (when second and next ones are not fallback generators). It is a clear limitation to local planner design.

What was done?

I've added extra parameter to the SimpleScoredSamplingPlanner constructor so user can actually select if, e.g., the second generator is the fallback one or should always be checked (just like the primary generator).

Does it affect API of the previous applications?

No, because:

• default value is set to false which does not change behavior compared to the current version
• extra constructor argument is equipped with default value so API of the present applications does not change