Add configurable datum LLH transformation using PROJ - #104

Simplify LlhTransformer thread safety

Create the PROJ state once in Init() and use a mutex for the transforms,
instead of per-thread PROJ state. Only the driver worker thread transforms
coordinates, so per-thread state is not necessary.

Use SDK Transformer for datum LLH, remove LlhTransformer

The SDK Transformer now has the options, CRS checks and thread safety that
LlhTransformer had. The driver keeps only the policy: InitLlhTrafo() checks
the CRS types, EcefToLlhRad() falls back to the built-in WGS84 conversion
if datum LLH is disabled. Without PROJ (FPSDK_USE_PROJ=0) the driver uses
the built-in WGS84 conversion and rejects datum LLH enabled. The driver no
longer links PROJ directly.

Use BUILD_TESTING for fixposition_driver_lib tests

Use include(CTest) and BUILD_TESTING (default ON) instead of
FPSDK_BUILD_TESTING. Build the tests only if GTest is available.

Load ROS1 datum_llh params only if the namespace exists

If datum_llh is present, all its params are required. If it is absent, the
defaults stay. Remove LoadOptionalRosParam().