Changelog:

- changed license: BSD+Patents -> MIT
- propagates exceptions raised in sub-indexes of IndexShards and IndexReplicas
- support for searching several inverted lists in parallel (parallel_mode != 0)
- better support for PQ codes where nbit != 8 or 16
- IVFSpectralHash implementation: spectral hash codes inside an IVF
- 6-bit per component scalar quantizer (4 and 8 bit were already supported)
- combinations of inverted lists: HStackInvertedLists and VStackInvertedLists
- configurable number of threads for OnDiskInvertedLists prefetching (including 0=no prefetch)
- more test and demo code compatible with Python 3 (print with parentheses)
- refactored benchmark code: data loading is now in a single file

The GPU compilation fails with g++ 8.3.1 and cuda 10.1
With this proposed fix the compilation works locally

Contributes to #751

+ Add conda packages metadata (now building Faiss using conda's toolchain);
+ add Dockerfile for building conda packages (for all CUDA versions);
+ add working Dockerfile building faiss on Centos7;
+ simplify GPU build;
+ avoid falling back to CPU-only version (python);
+ simplify TravisCI config;
+ update INSTALL.md;
+ add configure flag for specifying target architectures (--with-cuda-arch);
+ fix Makefile for gpu tests;
+ fix various Makefile issues;
+ remove stale file (gpu/utils/DeviceUtils.cpp).

Makes sense.

Facebook sync (Mar 2019)

- MatrixStats object
- option to round coordinates during k-means optimization
- alternative option for search in HNSW
- moved stats and imbalance_factor of IndexIVF to InvertedLists object
- range search for IVFScalarQuantizer
- direct unit8 codec in ScalarQuantizer
- renamed IndexProxy to IndexReplicas and moved to main Faiss
- better support for PQ code assignment with external index
- support for IMI2x16 (4B virtual centroids!)
- support for k = 2048 search on GPU (instead of 1024)
- most CUDA mem alloc failures throw exceptions instead of terminating on an assertion
- support for renaming an ondisk invertedlists
- interrupt computations with ctrl-C in python

Adding Code of Conduct file

Remove stale CMake reference.

* Improve Arm64 support.

Make library compile and run on Arm64 8.2 (Nvidia Xavier arm+cuda)
`configure` is regenerated

* remove simd from utils.cpp

When data is being added to each index block, it needs to explicitly identify ids.

There we go

* Fix test_transfer_invlists.cpp

* Fix relative imports.

* Fix test_index_accuracy.py.

* Use default OSX version.

* Allow osx gcc6 build to fail.

* [c_api] minor tweak on Index_c header imports

* [c_api] Multi GPU constructs

- Add constructor & destructor for `FaissGpuMultipleClonerOptions`
- Add function `faiss_index_cpu_to_gpu_multiple`
- Add function `faiss_index_cpu_to_gpu_multiple_with_options`

* [c_api] fix doc comments in GpuClonerOptions

* [c_api] Improve cpu_to_gpu_multiple prototype

* [c_api] Improve cpu_to_gpu_multiple prototype

* [c_api] Add dtor definition for GpuMultipleClonerOptions

Add missing `\` in gpu Makefile

* Add GpuIndexBinaryFlat
* Add IndexBinaryHNSW