follow on to PR 992 - when running sampler algorithm fixed_param instead of hmc, record in Stan CSV header

[mitzimorris]

Summary:

Wrapper CmdStanPy parses the Stan CSV outputs. If the sampler was called on a model with no parameters and the user failed to specify algorithm=fixed_param, CmdStan will run fixed_param automatically; however the Stan CSV file header won't record this. Add another header comment line to the sampler file to record this.

Description:

see discussion in CmdStanPy issue here: https://github.com/stan-dev/cmdstanpy/issues/425#issuecomment-894531130

Current Version:

v2.27.0