Tally trigger

This is the OpenMC code with tally precision triggers and corresponding documentation. There are some changes in the tally_header file. This Pull request fulfills issue #264.
Derived types should be CamelCase without underscores ``_``. Change ``K_trigger`` to something like ``KeffTrigger``. (see OpenMC Style Guide: http://mit-crpg.github.io/openmc/devguide/styleguide.html#derived-types-and-classes)