generate_quantities - more intelligent reading of CSV header file

[mitzimorris]

Summary:

Generate quantities method currently has var hmc_fixed_cols which expects NUTS sampler output columns.
Instead, read CSV column header names; sampler variable names end in __ - calculate offset from this.
This will allow more inputs consisting of just parameter columns from a set of draws.

Description:

We want to make the use of generate_quantities method work in a production setting by allowing minimal CSV file which is still parse-able by Stan CSV reader, since we still need to allow full Stan CSV files as well.

Current Version:

v2.24.0

[WardBrian]

#1106