build_diffusion helper

Should be just about done refactoring construction
of PlanetPhysicsHelper
This didn't actually get used anywhere
Make parsing methods const
build_diffusion didn't need a GetPot object
Some compile time bug fixes
Some missing parsing methods
Fixes to build system
Fix from Sylvain for parsing
Need to initialize binary diffusion coeffs
Check for input file
Parsing fix from Sylvain
This will be moved to Evaluator

For now, don't try to resize it since we don't have it
Adding input file for new (forthcoming) test
.gitignore