Previously, if a `Node` was added to an `Executor` using the executors `add_node` method, then nodes `executor` property would return `None`.

Signed-off-by: Jacob Perron <jacob@openrobotics.org>

* Add Action Client (#262)

* Add rclpy_action module

With functions for creating and destroying action clients.

* Implement action client

* Move common conversion function and typedefs to shared header file (impl/common.h)

* Add tests using mock action server

* Add action module for aggregating action related submodules

* Extend Waitable API so executors are aware of Futures

* Move check_for_type_support() to its own module

Signed-off-by: Jacob Perron <jacob@openrobotics.org>

* Fix Executor not executing tasks if there are no ready entities in the wait set (#272)

If a task is created, then trigger the Executors guard condition. This will wake any blocking call to `rcl_wait()`.
In this scenario, no work is yielded, so we also have to move the check for 'in-progress' tasks into the wait loop.

Added a unit test to reproduce the issue.

This resolves an issue in some cases where the result response from the action server is never processed, therefore never reaching the action client.

Signed-off-by: Jacob Perron <jacob@openrobotics.org>

* Fix Node's reference to executor (#275)

Previously, if a `Node` was added to an `Executor` using the executors `add_node` method, then nodes `executor` property would return `None`.

Signed-off-by: Jacob Perron <jacob@openrobotics.org>

* Abstract type conversions into functions (#269)

* Abstract type conversions into functions

This helps with readability and maintainability.
Also eliminated use of assertions during conversions, defering to exceptions.

* Move common C functions to a shared library 'rclpy_common'

Signed-off-by: Jacob Perron <jacob@openrobotics.org>

* Add ActionServer (#270)

* Add Action server functions to extension module
    * Separated service related macros into separate request and response calls
* Add server goal handle functions to extension module
* Update Action extension module to use conversion functions
* Add implementation of Python ActionServer
    * Handles goal and cancel requests, responds, and calls user-defined functions for executing goals.
    * Handle result requests
    * Handle expired goals
    * Publish goal status array and feedback
* Add `handle_accepted_callback` to ActionServer

Upon accepting a goal, the provided `handle_accepted_callback` is called with a reference to the goal handle.
The user can then decide to execute the goal or defer.
If `handle_accepted_callback` is not provided, then the default behavior is to execute the goal handle immediately.

Signed-off-by: Jacob Perron <jacob@openrobotics.org>

* Enable test using MultiThreadedExecutor (#280)

Signed-off-by: Jacob Perron <jacob@openrobotics.org>

* Guard against failed take when taking action messages (#281)

Some middlewares (e.g. Connext and OpenSplice) send invalid messages to indicate an instance has been disposed which results in a 'failed take'.

Signed-off-by: Jacob Perron <jacob@openrobotics.org>