Reference elem

For the cut cell quadrature subintegration, it would be nice to have singletons that provide acces to the reference cell for each element type.  That is, a full-featured Elem that is posed on the reference domain.  Consider for example:

```c++
Elem *phys_elem;

Elem const * ref_elem = phys_elem->reference_elem();
```

This patchset implements the singleton objects.  They are read from static strinstream data to avoid disk I/O, and the singleton cache is updated only once, at first access.

Opening this up for comments before I add the Elem interface and implement the missing edges.

-Ben