Make sure the code compiles with vectorization switched on and using a different hydro scheme from Gadget-2.

This reverts commit ab51e592.

Created a branching function for DOPAIR2 to only call the vectorised function when vectorisation is enabled and call the serial version when corner cells interact. Moved some debug statements inside the branching functions.

runner_dopair2_force_vec now runs correctly without looping over all particles. max_index have been populated using the max between hi and hj_max.

Fixed bug where max_index_i could be count_j instead of count_j - 1. Also fixed bug where an exit iteration of 0 prevents the inner loop from executing. For dopair2.