Updated the license from GPLv2+ to LGPLv2.1+ for my systems in system.

[andgi]

I want to update the license from GPLv2+ to LGPLv2.1+ for my systems in systems. The systems are {airship-mooring,airship_added_mass,hydrodynamics,hydrodynamic-planing,hydrodynamic-planing-floats,visual-reference-point-extensions}.xml.

As far as I can see I am the only one contributing to these here and in FlightGear.

Cheers, Anders

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (952ef8d) 24.87% compared to head (998d122) 24.87%. Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1001 +/- ## ======================================= Coverage 24.87% 24.87% ======================================= Files 168 168 Lines 18908 18908 ======================================= Hits 4704 4704 Misses 14204 14204 ```

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