Hex areas and use for Deployment, Flee Zones and Triggers

[SJuliez]

This PR continues my work to implement some (pretty simple) Kell Hounds scenarios from the old Kell Hounds book as MM scenarios with a bit of control. Changes:

• adds hex areas; they come in various basic forms and can be combined by adding, subtracting and intersecting to form more complex areas; hex areas are always evaluated against a board to determine if they contain a hex or to get all their coords; currently implemented areas include geometric ones like hex lines, half planes, circles, rays; others are board-dependent such as the board edges or hexes with a given terrain or hex level; hex areas are lightweight and good for transfer between server and client as they don't store hexes, only rules to create them;
• hex areas can be used to set deployment areas via code; it uses the same system as the terrain-type deployment zones
• players and individual units can now be given a hex area as a flee zone from which they are allowed to leave the battlefield; this can be anywhere, even in the middle of the board; the default behavior is unchanged from before (units dont have their own zone and players use the board edges); there is now a menu item to show the flee zone of a unit; note that Princess currently makes no use of this
• hex areas can be used to trigger events when one or more units are within a hex area
• MM scenario parsing for the above
• limits fleeing in Lowering the Boom and Saving A Prince scenarios in accordance with scenario rule

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 29.06%. Comparing base (497e040) to head (db3216f).
Report is 50 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #6045      +/-   ##
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- Coverage     29.09%   29.06%   -0.04%     
- Complexity    13972    13981       +9     
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  Files          2580     2601      +21     
  Lines        267172   267592     +420     
  Branches      47710    47792      +82     
============================================
+ Hits          77744    77771      +27     
- Misses       185530   185919     +389     
- Partials       3898     3902       +4     

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[HammerGS]

Tested and works, just waiting for Tex to confirm the Javadoc bit.

[rjhancock]

/home/runner/work/megamek/megamek/megamek/megamek/src/megamek/common/hexarea/RectangleHexArea.java:36: error: malformed HTML

Replace the < with & lt; (without the space - html entity for less than sign) to fix the Javadoc issue