* Fixes codeplaysoftware#41 and codeplaysoftware#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.