writeAddSubtractField.C: explicitly dereference tmp to keep clang3.2 happy

Also remove the "<thermo" part of the names of thermodynamics packages

- allow both generic and version-specific. Only added to the path if
  the directory already exists when the bashrc/cshrc is sourced

STYLE: refactor (simplify) site-directory handling

To support these changes the need for "Sp" corrections on div-terms has been
eliminated by introducing a "bounded" convection scheme which subtracts the Sp
term from the selected scheme.  The equivalent will be needed for the ddt term.

A warning message is generated for steady-state solvers in which the "bounded"
scheme is not selected for the convection terms.