Support for different rain datarefs

[nico87]

Some X-Plane plugins, such XEnviro, use sim/weather/rain_percent instead of sim/weather/precipitation_on_aircraft_ratio. Directly providing the dataref as const char* via parameter/struct was not possible as fdr_find only accept literals. Moreover, the dataref must be a ratio. That is, some kind of validation would be required.

• A new librain_init_opt export has been added
• Using the librain_opt_t struct it is now possible to switch the rain dataref in use
• Ability to support more rain datarefs in the future
• librain_init works like before; no compatibility issues are expected with present implementations