Throw an exception if someone initializes an FEValues object with a quadrature formul...

Throw an exception if someone initializes an FEValues object with a quadrature formula that has no quadrature points. This can happen, more by accident, if one uses fe.unit_support_points as quadrature points and happens to try to do that with a non-interpolating element such as FE_DGP. If one later accesses fe_values.shape_value(i,q) for q=0...dofs_per_cell the resulting error message is completely unintelligible (it complains about an out-of-bounds access to some strange table). Better catch these sorts of cases up front.

git-svn-id: https://svn.dealii.org/trunk@25408 0785d39b-7218-0410-832d-ea1e28bc413d