Add Context and Endpoint classes to enable non-Communicator use-cases (#166)

This PR implements and closes #137. The new `Endpoint` and `Context`
classes expose the connection establishing functionality from
`Communicator`, which now is only responsible for tying together the
bootstrapper with a context.

The largest breaking change here is that
`Communicator.connectOnSetup(...)` now returns the `Connection` wrapped
inside a `NonblockingFuture`. This is because with the way `Context` is
implemented a `Connection` is now fully initialized on construction.

Some smaller breaking API changes from this change are that
`RegisteredMemory` no longer has a `rank()` function (as there maybe no
concept of rank), and similarly `Connection` has no `remoteRank()` and
`tag()` functions. The latter are replaced by `remoteRankOf` and `tagOf`
functions in `Communicator`.

A new `EndpointConfig` class is introduced to avoid duplication of the
IB configuration parameters in the APIs of `Context` and `Communicator`.
The usual usage pattern of just passing in a `Transport` still works due
to an implicit conversion into `EndpointConfig`.

Miscellaneous changes:
-Cleans up how the PIMPL pattern is applied by making both the `Impl`
struct and the `pimpl_` pointers private for all relevant classes in the
core API.
-Enables ctest to be run from the build root directory.

python/test/mscclpp_group.py

@@ -78,6 +78,7 @@ class MscclppGroup:
         for rank in remote_ranks:
             connections[rank] = self.communicator.connect_on_setup(rank, 0, transport)
         self.communicator.setup()
+        connections = {rank: connections[rank].get() for rank in connections}
         return connections
 
     def register_tensor_with_connections(