Include explicit stan version in reference posterior computations

[MansMeg]

Now only rstan version is included in the reference posterior. Since pystan might not follow stan versions, we should add an additional slot for stan version in all reference posterior computations.

Current structure:

    "rstan_version": "rstan 2.21.1",
    "r_Makevars": ... 
    "r_version": ...
    "r_session":  ...
    "r_summary_statistic": "posterior R package, version 0.1.2"

Proposed structure:

    "stan_version": "2.21",
    "interface_version": "rstan 2.21.1",
    "r_Makevars": ... 
    "r_version": ...
    "r_session":  ...
    "r_summary_statistic": "posterior R package, version 0.1.2"

Functionality to use:
https://mc-stan.org/rstan/reference/stan_version.html