Add test for the instantiation of deplete.Operator

Construct a minimal cross sections xml file that only
contains the depletion_chain node required by
the Operator. The path to this file is set to the
OPENMC_CROSS_SECTIONS file, and reverted after the test.

The depletion_chain points towards the chain_simple.xml
files, and a reference Chain is produced in the test.
The test involves creating a bare Operator instance, that
constructs a chain based on the temporary
OPENMC_CROSS_SECTIONS file. This chain is compared to
that produced by reading the chain_simple.xml file stored
in the test directory.

tests/unit_tests/test_deplete_operator.py

@@ -0,0 +1,72 @@
+"""Basic unit tests for openmc.deplete.Operator instantiation
+
+Modifies and resets environment variable OPENMC_CROSS_SECTIONS
+to a custom file with new depletion_chain node
+"""
+
+from os import remove
+from os import environ
+from unittest import mock
+from pathlib import Path
+import pytest
+
+from openmc.deplete.abc import TransportOperator
+from openmc.deplete.chain import Chain
+
+BARE_XS_FILE = "bare_cross_sections.xml"
+CHAIN_PATH = Path().cwd() / "tests" / "chain_simple.xml"
+
+
+@pytest.fixture(scope="module")
+def bare_xs():
+    """Create a very basic cross_sections file, return simple Chain.
+
+    """
+
+    bare_xs_contents = """<?xml version="1.0"?>
+<cross_sections>
+  <depletion_chain path="{}" />
+</cross_sections>
+""".format(CHAIN_PATH)
+
+    with open(BARE_XS_FILE, "w") as out:
+        out.write(bare_xs_contents)
+
+    yield
+    remove(BARE_XS_FILE)
+
+
+class BareDepleteOperator(TransportOperator):
+    """Very basic class for testing the initialization."""
+
+    # declare abstract methods so object can be created
+    def __call__(self, *args, **kwargs):
+        pass
+
+    def initial_condition(self):
+        pass
+
+    def get_results_info(self):
+        pass
+
+
+@mock.patch.dict(environ, {"OPENMC_CROSS_SECTIONS": BARE_XS_FILE})
+def test_operator_init(bare_xs):
+    """The test will set and unset environment variable OPENMC_CROSS_SECTIONS
+    to point towards a temporary dummy file. This file will be removed
+    at the end of the test, and only contains a
+    depletion_chain node."""
+    # force operator to read from OPENMC_CROSS_SECTIONS
+    bare_op = BareDepleteOperator(chain_file=None)
+    act_chain = bare_op.chain
+    ref_chain = Chain.from_xml(CHAIN_PATH)
+    assert len(act_chain) == len(ref_chain)
+    for name in ref_chain.nuclide_dict:
+        # compare openmc.deplete.Nuclide objects
+        ref_nuc = ref_chain[name]
+        act_nuc = act_chain[name]
+        for prop in [
+                'name', 'half_life', 'decay_energy', 'reactions',
+                'decay_modes', 'yield_data', 'yield_energies',
+                ]:
+            assert getattr(act_nuc, prop) == getattr(ref_nuc, prop), prop