* Use structured bindings where noted (there are likely other opportunities in the code).
* Use `if constexpr` to simplify some template-dispatched code.
* Remove `arb::uitl::get`, as it is no longer required now that util::variant is replaced.
* Add `_v` utility inline constexpr template values.

Part of the C++17 migration task #1022.

* Substitute `std::optional<T>` for `arb::util::optional<T>` for non-reference types T.
* Remove reference-deducing `util::value_by_key`; replace usages with new function `util::ptr_by_key`.
* Add some missing header includes that were required but included only transitively.
* Remove `operator<<` overload for optional in python/strprintf.hpp; replace with utility wrapper class that catches `std::optional<T>`.
* Wrap some `std::optional` values with `to_string` in ostream output in python lib.

Fixes #1154.

* Update CMake CXX version.
* Make CUDA 10 a minimum requirement and remove CUDA 9 support code.
* Set up g++ Travis tests to use g++ 8.
* Explicitly set C++14 version for nvcc.
* Properly split cuda compilation, including in unit tests.
* Remove unnecessary modcc `SOLVE` warning.
* Update pybind11 module to tag v2.5.0
* Replace `util::size` and `util::data` with `std::` equivalents.

- Add option to compile Arbor for HIP/CUDA backend using Clang. 
- Add new CMake option `ARB_GPU_COMPILE_TYPE` to distinguish between three possible gpu platforms/builds: `cuda-nvcc`; `cuda-clang`; `hip-clang`
- Add gpu wrapper functions in `arbor/backends/gpu/gpu_api.hpp` and `arborenv/gpu_api.hpp` which call HIP/CUDA functions depending on the platform
- Rename functions and files: cuda -> gpu
- Add downgraded warp primitives for HIP
- Implement `uuid` workaround for HIP
- Set correct alignment and warp size for AMD gpus
- Update installation guide and docs.
- Update pip/setuptools to support new gpu targets

Fixes #833

Update the installation guide to reflect the latest supported tool,
compiler and library versions.

Remove the Python docs, because they documented features that have not
been implemented yet. The Python docs can be added incrementally as
features are implemented.

Start work on oupdating the documenetation for hardware interfaces and
domain decomposition.

Sam Yates authored 6 years ago and Benjamin Cumming committed 6 years ago

Make a new installed library `libarborenv.a` covering a subset of the `sup` library functionality, with corresponding installed CMake target `arbor::arborenv`.

* Move NVML or CUDA 10 API decision for GPU UUID discovery to top level CMake.
* Move affinity, concurrency, MPI init guard, and gpu detection and negotiation functionality out of `sup` and into new library `arborenv`.
* Move `include/arbor` in project tree to `arbor/include/arbor` (for consistency across `sup`, `arbor`, and `arborenv` subdirectories.)
* Wrangle more explicit library dependency adding CMake code into the installed `arbor-config.cmake`, to help mitigate [CMake issue #18614](https://gitlab.kitware.com/cmake/cmake/issues/18614).
* Have `arborenv` code throw `std::runtime_exception` instead of `arb::arbor_error`. (We are still using `arb::mpi_error` though for a failure in `with_mpi`.)
* Move `scope_exit` into the `arb::util` namespace.
* Merge `affini...