get_pa_competence_score: Compute a physical activity competence score.
Description
This function computes a physical activity competence score (pa_competence_score) for responses to items 4-6 of the the Behavioral Regulation in
Exercise Questionnaire (BREQ) as they appear in the CAPL-2 Questionnaire. This score is used to compute the motivation and confidence domain score
(mc_score).