templated scalar type and propto in generated log_prob

[Bob Carpenter]

The log_prob function is now templated, so you can specify:

-- the type of the scalar, such as double or agrad::var, and

-- the propto, so that you can either drop or not drop
normalization constants

I still need to do a bit of work until the log_prob<double,...>
version actually works. But everything that used to work now
works in the new setup.

- Bob