Feature/issue 470 log prob fun

#### Summary:

Adds `get_lp()` function to access accumulated log prob

#### Intended Effect:

Adding this function allows users to get the value of `lp__` plus the sum of the accumulator through a function call.  This is primarily for print diagnostics.

#### How to Verify:

There are unit tests and you can see the code generated for it.

#### Side Effects:

None.

#### Documentation:

There's a new paragraph on use `get_lp()` for debugging.  Then there's the doc of how it works in the functions part of the reference manual.

#### Reviewer Suggestions:

Anyone.