Add spack package file to the repository (#1466)

Replicate the spack `package.py` from the main spack repository into the arbor repo.

Some small updates were made:
- Correctly provide a url for the GitHub tagged release
- Specify minimum pybind11 version 2.6

Fixes #1465 

spack/package.py

+# Copyright 2013-2021 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+
+from spack import *
+
+
+class Arbor(CMakePackage, CudaPackage):
+    """Arbor is a high-performance library for computational neuroscience
+    simulations."""
+
+    homepage = 'https://arbor-sim.github.io/'
+    git      = 'https://github.com/arbor-sim/arbor.git'
+    url      = 'https://github.com/arbor-sim/arbor/releases/download/v0.5/arbor-v0.5-full.tar.gz'
+    maintainers = ['bcumming', 'brenthuisman', 'haampie', 'halfflat']
+
+    version('master', branch='master', submodules=True)
+    version('0.5', 'd0c8a4c7f97565d7c30493c66249be794d1dc424de266fc79cecbbf0e313df59', url='https://github.com/arbor-sim/arbor/releases/download/v0.5/arbor-v0.5-full.tar.gz')
+
+    variant('assertions', default=False, description='Enable arb_assert() assertions in code.')
+    variant('doc', default=False, description='Build documentation.')
+    variant('mpi', default=False, description='Enable MPI support')
+    variant('neuroml', default=True, description='Build NeuroML support library.')
+    variant('python', default=True, description='Enable Python frontend support')
+    variant('vectorize', default=False, description='Enable vectorization of computational kernels')
+
+    # https://arbor.readthedocs.io/en/latest/install/build_install.html?highlight=requirements#compilers
+    conflicts('%gcc@:8.3')
+    conflicts('%clang@:7')
+    # Cray compiler v9.2 and later is Clang-based.
+    conflicts('%cce@:9.1')
+    conflicts('%intel')
+
+    depends_on('cmake@3.12:', type='build')
+
+    # misc dependencies
+    depends_on('nlohmann-json')
+    depends_on('cuda@10:', when='+cuda')
+    depends_on('libxml2', when='+neuroml')
+
+    # mpi
+    depends_on('mpi', when='+mpi')
+    depends_on('py-mpi4py', when='+mpi+python', type=('build', 'run'))
+
+    # python (bindings)
+    extends('python', when='+python')
+    depends_on('python@3.6:', when="+python", type=('build', 'run'))
+    depends_on('py-numpy', when='+python', type=('build', 'run'))
+    depends_on('py-pybind11@2.6:', when='+python', type=('build', 'run'))
+
+    # sphinx based documentation
+    depends_on('python@3.6:', when="+doc", type='build')
+    depends_on('py-sphinx', when="+doc", type='build')
+    depends_on('py-svgwrite', when='+doc', type='build')
+
+    @property
+    def build_targets(self):
+        return ['all', 'html'] if '+doc' in self.spec else ['all']
+
+    def cmake_args(self):
+        args = [
+            self.define_from_variant('ARB_WITH_ASSERTIONS', 'assertions'),
+            self.define_from_variant('ARB_WITH_MPI', 'mpi'),
+            self.define_from_variant('ARB_WITH_NEUROML', 'neuroml'),
+            self.define_from_variant('ARB_WITH_PYTHON', 'python'),
+            self.define_from_variant('ARB_VECTORIZE', 'vectorize'),
+        ]
+
+        if '+cuda' in self.spec:
+            args.append('-DARB_GPU=cuda')
+
+        # rely on spack's compiler wrapper to set architecture
+        args.append('-DARB_ARCH=')
+
+        return args