Initial commit of reference configuration manager.

Maintain a field x_rc, the reference configuration for the solution x, that is
updated x_rc += x (and then interpolated to the new mesh) at the time of mesh
adapatation.

If this capability is not explicitly turned on in the parameter file, it does
nothing. It is turned on as follows:
  <ParameterList name="Adaptation">
    <Parameter name="Reference Configuration: Update" type="bool" value="true"/>
  </ParameterList>
Restoring ENABLE_MESHMODEL to CMakeLists.txt for SCOREC.

The definition got lost by mistake (I'm pretty sure it was not intended) in a
recent SCOREC config change. The result was that SCOREC ctests were not turned
on. Now fixed. Apologies if this was an intended change that I just reverted.