fix ppf scaling to ensure labour share in GDP

• the share of labour in GDP should be at least 20 % in historic periods (t_29hist), and non-negative (set to > 0.5 %) in all other periods
• scaling ppf prices as soon as quantities and prices are established in the calibration, and before any other parameters are derived, circumvents inconsistency problems in those calculations
• the addionional, later, check and scaling is left in for now to allow for testing if it is still triggered