Fix #5795: Implement TW LOS and Forest rules for Low Altitude maps

[Sleet01]

It looks like this has been broken since before 0.48.0, maybe since 2003: we have been treating Low-Altitude map terrain as if it were Ground Map terrain, which means it never obscures Aerospace units from each other.

Buildings and landed Dropships (which are treated as buildings) should not obscure lines of site between any Aero units using Altitude on a Low-Altitude map.
Additionally, forests should only rise 1 level above the terrain they are on when Low Altitude maps are created from Ground Maps - thankfully the Hex class provides a mechanism to tweak this stat for forest / jungle / foliage hexes.

Total Warfare p. 81 says:

Each level on a ground mapsheet hex corresponds to an
aerospace altitude. For example, a hill marked Level 2 equals
Altitude 2. Units can enter that hex only if flying at Altitude 3
or higher (or 2 if landing; see p. 87). Wooded hexes rise one
altitude above the underlying terrain. For simplicity sake,
building hexes do not rise above the underlying terrain and
are ignored by aerospace units (unless the unit is attempting
to land, see p. 87).

Unfortunately it was much simpler to reverse the normal conversion and use the units' Altitudes as Elevation, rather than convert Elevation into Altitudes, due to the way that LosEffects is written.

Testing:

• Various ASFs and Conventional Fighters on a multitude of Low Altitude maps constructed from existing templates.
• Ran all 3 projects' unit tests.

Close #5795

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 29.05%. Comparing base (8b06aec) to head (6827413).
Report is 16 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5797      +/-   ##
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- Coverage     29.06%   29.05%   -0.01%     
+ Complexity    13913    13908       -5     
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  Files          2507     2507              
  Lines        266339   266362      +23     
  Branches      47611    47624      +13     
============================================
- Hits          77403    77397       -6     
- Misses       185012   185027      +15     
- Partials       3924     3938      +14     

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[SJuliez]

I have some comments.