Remove leading underscores from src/stan/mcmc classes and corresponding ...

#### Summary:

Move leading underscores on private class members/methods to suffix (issue #776).

#### Intended Effect:

Change of internal names, no external effects. 

#### How to Verify:

Run any model -- there should be no effects.

#### Side Effects:

None user facing.  Corresponding unit tests had to be updated.

#### Documentation:

None.

#### Reviewer Suggestions:

Anyone.