Add better airway support, fix various issues with VIA, and add
parsing / generation of ICAO route strings. Also fix the
serialisation of flight-plans with airway enroute segments, so these
can be restored correctly.
The test_suite/helpers directory has been renamed to test_suite/FGTestApi/ and
all the fgtest namespaces renamed to FGTestApi. The helper functions are now
also placed into either a setUp or tearDown namespace depending on their
function. And a new FGTestApi::PrivateAccessor::FDM namespace has been created
for holding friend classes used to access private or protected variables or
functions from solely within the test suite.
