Adding LinearCombinationAux

LinearCombinationAux is a kernel that computes a linear combination of
two vectors of variables, i.e. u = \Sum a_i * b_i

Closes #4281
Merge pull request #4286 from andrsd/lin-comb-aux

Adding LinearCombinationAux
Merge commit 'ad5606d4ac25a0369e1539584baa429d24a2c7f1'