`iter_warmpup=0` and `fixed_param=True` will always fail for stan 2.34.1

[carrascomj]

Summary:

If fixed_param=True, adapt_engaged will always be left as default (True) because of this early return https://github.com/stan-dev/cmdstanpy/blob/develop/cmdstanpy/cmdstan_args.py#L361.

This is okay for the newer stan since this if-statement. But for previous stan (tried 2.34.1), this will always fail since adapt_engaged must be False even when fixed_param is supplied.

Description:

This only affects previous versions of Stan. I am not sure about the policy of supporting versions but it is a breaking change that will make previously working code fail if cmdstan itself is not updated.