Issue #41: Separate execution and memory resources

Fixes #41 and #42.
Introduce memory_resource to represent the memory component of a system topology.
Remove can_place_memory and can_place_agents from the execution_resource as these are no longer required.
Remove memory_resource and allocator from the execution_context as these no longer make sense.
Update the wording to describe how execution resources and memory resources are structured.
Refactor affinity_query to be between an execution_resource and a memory_resource.
Make minor corrections.

codeplaysoftware / standards-proposals