Changes

distances: support distances between objects whose types are in
multiple levels
Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>

distances: allow inserting distance matrices between objects of
different types
This may be used for upcoming NVLink distances between GPUs and CPUs, 
or roofline-like distances between PUs/Caches/Memories.
Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>

xml/distances: add support for heterogeneous distances
Use a new "distances2hetero" tag instead of adding hetero-specific
attributes to the existing "distances2" tag. The latter would fail to
load entirely in pre-2.1 releases, while the former just ignores the
remaining of the XML (not an issue since the new tag is always after
everything supported in pre-2.1).
Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>

tests: add a heterogeneous distances in tests/hwloc/hwloc_distances.c
Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>

tests/xml: add a test with heterogeneous distances
Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>

distances: hwloc_distances_get_by_type() isn't inline on top of
by_depth() anymore
Make it work even for Groups etc.
Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>