Singletons for each reference element type.

This patch implements singleton objects for each reference element type.  At presenent they are read from our "reference_elements/" files, but that does not scale.  My next attempt will be to embed those definitions automatically through static stringstream objects.

This is an incrimental feature to support the following usage:

Elem *phys_elem;

Elem const * ref_elem = phys_elem->reference_elem();

where phys_elem is a typical element and ref_elem is its reference element in parametric coordinates.
shell script to create static reference element strings from files.
include *.data in the source file list
reference element data
implement reference element data
implements reading from static strings rather than file
fix generated file
remove temporary cruft
newline
return a reference per Roys suggestion
implement missing 1D reference elements
predictable sorting
implement remaining reference elements
reimplement using Singleton
remove redundant test
remove print statement
reimplement using Singleton::Setup
Elem::reference_elem()
defer initialization until requested
clarify comment