Rebuild services using config objects with defaults

[bob-carpenter]

Summary:

The service functions in stan/services are too hard to invoke because every argument must be supplied and there are over 20 of them for each service method.

Description:

Replace the existing service function design with a new design based on multiple inheritance of base configuration classes with defaults, validation and pretty printing, which may be shared among services. This keeps everything in straight C++ code, with no dynamic casts required, and allows the client to easily specify only the non-default parameters.

For design discussion and sketch, see this forum thread.

Current Version:

v2.18.0