Merge branch 'fix_pair_vec' into 'master'

 - Use the correct value of 'h_max' in the construction of the caches in the vectorized DOPAIR1 routine. (fix #312)
 - Update the test27 unit test to also randomly perturb the smoothing length values.
 - Set a correct value for h_max in the test27 case and not just a loose upper-bound.
 - Call the non-vectorized version of DOPAIR1 for the corner interactions.

See merge request !348

EAGLE_25 now produces the same number of time-steps whether running with or without vectorization.