Added NDPluginCircularBuff

Allows triggering based on an attribute flag in NDArrays.

Supports pre- and post-trigger frames.

It works in the following way at present:

* Arm plugin
* It starts filling the circular buffer with "pre" images
* When it gets an image with an attribute named "EXT_TRIGGER" with a non-zero value it outputs the contents of the circular buffer
* For the next "post" images, just output them
* Wait for a re-arm