Surface-distance license and attribution

RTpipeline includes Google DeepMind's surface-distance code under the Apache License 2.0. Copyright 2018 Google Inc. All Rights Reserved.

Source: google-deepmind/surface-distance, commit 1f805cea446805e559badc80dd27f34175e4173d.

The package includes the upstream __init__.py, metrics.py, lookup_tables.py and LICENSE, plus RTpipeline's NOTICE, in rtpipeline/eval/_vendor/surface_distance/. The upstream files are unchanged. The NOTICE identifies the source and RTpipeline's pooled mean-distance calculation. See surface metric definitions.