Improve handling of initial values for hierarchical distributions

[gowerc]

Several parameters have their distributions fixed via the hierarchical distribution for example:

    lm_gsf_psi_bsld ~ lognormal(lm_gsf_mu_bsld[pt_study_index], lm_gsf_omega_bsld);  
    lm_gsf_psi_ks ~ lognormal(lm_gsf_mu_ks[pt_arm_index], lm_gsf_omega_ks);  
    lm_gsf_psi_kg ~ lognormal(lm_gsf_mu_kg[pt_arm_index], lm_gsf_omega_kg);  
    lm_gsf_psi_phi ~ beta(lm_gsf_a_phi[pt_arm_index], lm_gsf_b_phi[pt_arm_index]);  

Currently our framework sets their initial value via prior_none() which then uses Stans default process of drawing an initial value from a Uni(-2, 2) distribution.

prior_none() is required in order to get the current code to create an initial value without inserting an additional prior Stan statement. Potentially as a future improvement it would be good to remove prior_none() entirely and enable an additional argument to suppress inclusion of the Stan code from the existing prior_x() functions so that you can set initial values more easily.