Fixed a possible bug in VectorMassIntegrator::AssembleElementMatrix
Fixed a possible bug in VectorMassIntegrator::AssembleElementMatrix

VectorMassIntegrator::AssembleElementMatrix got an information about
the dimension of the vector problem based on the dimension of the vector
or matrix coefficient. If the integrator is supposed to use a scalar
coefficient the dimension was equal to the dimension of the finite element,
which is okay if it's a domain integration, but is wrong when it's
integration on a boundary.

mfem/mfem,kvoronin/mfem-4d-own,mfem/mfem,martemyev/mfem,mfem/mfem,gelever/lamfem,mfem/mfem,mfem/mfem