Bugfix/302 assign random variation to mcs pcs life satisfaction scores

[andrewbaxter439]

Bringing in PR for comparison and checks just now, though do hold off merging until refactoring sorted.

What

• adds expected variation to MCS, PCS, Life Satisfaction and DHE variables
• Restricts age of mental health and wellbeing processes to over 16 (for stage 1) and 25-64 (for stage 2) in line with calculated effect estimates

Why

• These were likely giving overly-precise mental health estimates and applying effects to wider-than-intended age groups

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