Reverse the loop to find the exit condition in DOPAIR2() to go from the centre to the outside

Matthieu Schaller requested to merge DOPAIR2_improvements into master

Follows @jwillis' suggestion in #362 (closed).

Also implements an early exit in the outer loop. If no particle can be in range any more (based on hi_max and hj_max), no need to keep checking the next particles.

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