Update Style Guide for template definitions

[eflumerf]

At the 4/24/2020 Application Framework meeting, we agreed to modify the DUNE style guide to require that non-trivial method definitions for template classes should go in a .icc file to be located in a subdirectory of the include directory where the template class's header is located, name TBD. This .icc file should be included in the template class header file after the class declaration.