Bugfix/issue 568 epil

## Summary

Fix #568 by changing the name of the template typename parameter T to T_ in the log_prob member function of a generated model class.

## Intended Effect

Avoid what appears to be a bug in gcc 4.2.1 on Mac OS X 10.7.5, in which a member variable T is confused with the template typename argument T, causing the epil model not to compile.

## Example

See issue 568.

## Side Effects

None.

## Documentation

None needed.

## Reviewer Suggestions