Only call vectorized routines when doing density/force

In runner_doiact.h the branching functions currently always call the vectorized density and vectorized force when we compile with vectorization switched on.

This is incorrect behaviour. The PAIR1 branching function should only call the vectorized density function when FUNCTION is set to density. For instance if I define FUNCTION to be gradient as is the case in more advanced hydro schemes, there is a function runner_dopair1_gradient() that exists and should be called. At the moment the branchning function named runner_dopair1_branch_gradient() calls runner_dopair1_density_vec() if vectorization is switched on (and compiling with gadget). It should be calling the non-vectorized and currently existing function runner_dopair1_gradient().

Some work with the macros will be required here.