Allow instantiation of CmdStanModel object from exe file only, no stan file

[mitzimorris]

This is allowed in CmdStanPy because there are a few compelling use cases.

Please document what a suitable workaround in CmdStanR is. Or implement.

[rok-cesnovar]

The current workaround is to create a dummy Stan file and change the modified time of the exe file.

There is an issue with doing this with any of the currently available CmdStan versions. Its that we have no idea whether a given exe was built with STAN_THREADS or any other flags.

We can ignore this and let the user worry about that and figure it out.

this should be fixed for 2.27

[mitzimorris]

We can ignore this and let the user worry about that and figure it out.

that's my feeling.

this should be fixed for 2.27

great - what exactly do you mean by "fixed"?

[rok-cesnovar]

great - what exactly do you mean by "fixed"?

We can now check whether the model was compiled with threading, opencl or MPI enabled with only the executable. So we know whether the threads environment variable needs to be set up, whether it needs to be called with MPI or whether the OpenCL device IDs need to be given.