Edit petsc/da examples and makefile.

 * Change code to remove warnings by the compiler.
 * Return PetscErrorCode when CHKERRQ is used, or use CHKERRV for void.
 * Compile all files at once.
Add file to demonstrate use of function pointer.
Add littleEndian.cpp.

* Add *.out to .gitignore.
* Remove unnecessary a.out.