Subpools are for grouping components by some criterion. Any component
that has a rangeid callback will be grouped with other components that
return the same render id. Note that the ordering of components within a
group will be affected by adding a component into a group that comes
before that group (or removing a component).
Component pools can have multiple groups, added and removed dynamically,
but removing a group should (currently) be done only when empty.
While this does require an extra call after registering components, it
allows for multiple component sets (ie, sub-systems) to be registered
before the component pools are created. The base id for the registered
component set is returned so it can be passed to the subsystem as
needed.
There's now a main ecs.h file that includes the sub-system headers,
removing the need to explicitly include several header files, but the
sub-systems are a less cluttered.
