Switch projectOldCellOntoNewCells() in Solution to use Functions instead of the legacy AbstractFunction.  In order to enable this, reworked some of the NewBasisSumFunction, so that it can know where the basis is supposed to be defined physically.
Added optional constructor argument to Var, a LinearTerm that represents the field variable(s) traced by the Var.  The hope is to use this in Solution during refinements to give a faithful projection of the existing field coefficients onto the newly defined trace coefficients.