ConstraintComponent, various changes

- Rename CONSTRAINT_GROUP_TYPE_FROM_CONSTRAINT_COMPONENT to
CONSTRAINT_GROUP_TYPE_SCENE.
- Update documentation
- Reduce number of nullptr tests, use EXPECT_THROW instead of
EXPECT_ANY_THROW.
setConstraintGroup, optimization

- Avoid allocating new memory for each call of setConstraintGroup by
acquiring a reference to the vector and calling reserve.
- Because we call reserve, we could possibly face memory leaks.
Therefore we also call shrink_to_fit.