Add typing information for error_map init argument

simple_pid/PID.pyi

@@ -19,6 +19,7 @@ class PID(object):
         output_limits: _Limits = ...,
         auto_mode: bool = ...,
         proportional_on_measurement: bool = ...,
+        error_map: Optional[Callable[[float], float]] = ...,
     ) -> None: ...
     def __call__(self, input_: float, dt: Optional[float] = ...) -> Optional[float]: ...
     def __repr__(self) -> str: ...