To expose PrecisionModel through C-API

• Create a function GEOSPrecisionModel in geos_c.cpp which calls GEOSPrecisionModel_r in geos_ts_c.cpp to make it thread safe.
• GEOSPrecisionModel_r creates a new precision model with arguments (double newScale, double newOffsetX, double newOffsetY) with fixed precision(the arguments that need to be passed are to be finalized).
• Create a new geometry factory by passing this precision model to it.
• Create a function GEOSContext_setPrecisionModel which sets the geometry factory and precision model to be used in that context.
• Write test cases.