Overriding frame memory allocator

[EmilioPeJu]

Hi all,

I am working on an AD plugin which uses shared memory to pass the frames to an external process with minimal overhead, unfortunately NDArrayPool doesn't provide mechanisms to override the mapping/unmapping of memory:

• The hook functions don't have enough power to affect the memory management.
• Creating a NDArrayPool subclass wouldn't work as alloc/release methods are not virtual, and even if they were it would affect only the plugin instantiating it.

The approach I took is providing a function to override function pointers used in place of malloc/free inside NDArrayPool

If this approach is appreciated, see pull request #486, otherwise I'm open to alternative approaches