Added UQTk package (#14801)

* added UQTk package

* Add missing expat dependency to uqtk

var/spack/repos/builtin/packages/uqtk/package.py

+# Copyright 2013-2020 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 Uqtk(CMakePackage):
+    """Sandia Uncertainty Quantification Toolkit. The UQ Toolkit (UQTk) is a
+    collection of libraries and tools for the quantification of uncertainty
+    in numerical model predictions"""
+
+    homepage = "https://www.sandia.gov/UQToolkit/"
+    url      = "https://github.com/sandialabs/UQTk/archive/v3.0.4.tar.gz"
+    git      = "https://github.com/sandialabs/UQTk.git"
+
+    version('master', branch='master')
+    version('3.0.4', sha256='0a72856438134bb571fd328d1d30ce3d0d7aead32eda9b7fb6e436a27d546d2e')
+
+    depends_on('expat')