Path forward on IO operations #785

This issue serves to document plans that myself, @bob-carpenter, and @mitzimorris have all discussed in person.

Issues:

• the existing IO code re-uses very little between the different services.
• the existing IO code is slow.
• the existing IO code depends on things like the order of comments in the files.

Steps:

• Split IO code into two passes. One pass should ignore all comments and use a fast reader like numpy's fromtxt to read in the draws. This can be used for all services. The second pass should deal with the comments. This will be service-specific. This change can be done any time
• Replace the second pass of the above with the newer JSON outputs rather than CSV comments. This requires a minimum supported cmdstan version: Remove support for CmdStan pre-2.34 in a future version #685
• The first step could later be replaced with some sort of binary output. This would require upstream Stan changes