Simplify debugging for potential parser issue

[rok-cesnovar]

Cmdstan 2.22 uses the new parser (stanc3) and we should add a simple interface to switch to the older parser for debugging purposes.

That would simplify debugging issues like stan-dev/math#1643 that turned out to not be related to the parser.

compile() should accept something like (use_stanc2 = TRUE), that would be turned off by default. This should only be allowed for 2.22 and up until he version where the existing compiler will be phased out.

[mitzimorris]

great idea - filed stan-dev/cmdstanpy#202 - can argument be simply stanc2 instead of use_stanc2?

[rok-cesnovar]

Yeah, definitely. Also think this should print a warning that the flag is on and should only be used to debug.

[mitzimorris]

this isn't feasible as a flag on the compile function given current CmdStan makefiles - right now CmdStan assumes that there is only one compiler - bin/stanc - and it builds it either from Stanc2 sources or uses the release or nightly build binary.
putting this flag on the compile function assumes that both compilers are available. will file an issue on CmdStan to make this happen.

[mitzimorris]

stan-dev/cmdstan#819

[rok-cesnovar]

I am going to close this one for two reasons:

• we cant do this for 2.22 as that needs a make clean and build
• will be fixed by addressing simplify/generalize model compilation options #121