ADD recipes for SciUnit, Cerebrus, Quantities-SciDash

Includes recipe for SciUnit (and its dependencies)

packages/py-cerebrus/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 PyCerebrus(PythonPackage):
+    """Lightweight, extensible schema and data validation tool for Python dictionaries
+    """
+
+    homepage = "http://docs.python-cerberus.org/"
+    pypi     = "Cerberus/Cerberus-1.3.4.tar.gz"
+    git      = "https://github.com/pyeve/cerberus.git"
+
+    version('1.3.4', sha256='d1b21b3954b2498d9a79edf16b3170a3ac1021df88d197dc2ce5928ba519237c')
+
+    depends_on('python@3.6.9:')
+    depends_on('py-setuptools',     type=('build', 'run'))
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