Issue 6271: Offboard units to the North and West will be placed properly

[psikomonkie]

Fixes #6271

Offboard units to the North and West will be placed properly; use one method for determining artillery travel time

Units placed to the South and East will be placed at coordinates (0 indexed!) map-dimension (1-indexed!) + offboard distance.

Units placed to the North and West were placed at coordinates (0 indexed!) -offboard distance.

This resulted in units to the North and West being one hex closer than they should be, allowing users to fire artillery at less than a mapsheet despite being offboard, which shouldn't be possible. This was already accounted for with the South and East because they use the 1-indexed map dimension.

I also noticed an identical artillery flight time algorithm in Entity.java and ArtilleryAttackAction.java. The one in Entity was already a method, so I changed ArtilleryAttackAction.java to use that method rather than implementing it on its own.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 28.95%. Comparing base (3b7ac02) to head (837ceda).
Report is 17 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #6308      +/-   ##
============================================
- Coverage     28.95%   28.95%   -0.01%     
+ Complexity    14032    14031       -1     
============================================
  Files          2672     2672              
  Lines        269292   269296       +4     
  Branches      47929    47934       +5     
============================================
- Hits          77970    77969       -1     
- Misses       187418   187426       +8     
+ Partials       3904     3901       -3     

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