Do not run the Richardson Extrapolation error scaling tests by default because they take forever.

They can still be run by specifying them explicitly, e.g.:
./run_tests.py integrators/richardson_extrapolation/error_scaling

git-svn-id: svn+ssh://svn.code.sf.net/p/xmds/code/trunk/xpdeint@2906 41b19578-b77a-4e3b-9682-c8570d237623
Removing layers of pointer indirection in the Richardson Fixed Step integrator.

This should make it a bit faster, and certainly makes it more readable.

git-svn-id: svn+ssh://svn.code.sf.net/p/xmds/code/trunk/xpdeint@2907 41b19578-b77a-4e3b-9682-c8570d237623