Feature request: Add GetDynPressure routine to SeaState_C_Binding interface

[andrew-platt]

Some codes using SeaState, such as SEAHOWL need a method for retrieving the dynamic pressure. Hooks have been added in SEAHOWL for a routine:

    SeaSt_C_GetFluidDynP(             //
                         time,        // in  - double
                         Pos_C,       // in  - 3D position, float[3], absolute coordinates
                         DynP_C,      // out - Dynamic pressure, float (N/m^2) 
                         ErrStat,     // out - Error status (0: none, 1: Info, 2: warn, 3: severe, 4: fatal)
                         ErrMsg);     // out - Message returned about error (empty if none)