New plugin hook for before reactor construction

[opotowsky]

What is the change?

This adds a new plugin hook beforeReactorConstruction. Then some code (material namespace order setting) is pulled out of the reactor factory and into the hook call for the reactor plugin.

Why is the change being made?

This cleans up the reactor construction and sets the stage for plugins to fancify the beforeReactorConstruction hook should they be interested in fancification.

---

Checklist

• This PR has only one purpose or idea.
• Tests have been added/updated to verify any new/changed code.

• The code style follows good practices.
• The commit message(s) follow good practices.

• The release notes have been updated if necessary.
• The documentation is still up-to-date in the doc folder.
• The dependencies are still up-to-date in pyproject.toml.

[ntouran]

Here's a patch that will get at least some test coverage. It's not easy to confirm positively that the reactor doesn't exist because the plugin hook has no knowledge of that. I think just showing that it's run during init is probably sufficient.

diff --git a/armi/tests/test_user_plugins.py b/armi/tests/test_user_plugins.py
index 0840afd5..0cc19f0d 100644
--- a/armi/tests/test_user_plugins.py
+++ b/armi/tests/test_user_plugins.py
@@ -12,6 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 """Tests for the UserPlugin class."""
+
 import copy
 import os
 import unittest
@@ -106,6 +107,11 @@ class UserPluginOnProcessCoreLoading(plugins.UserPlugin):
         for b in blocks:
             b.p.height += 1.0

+    @staticmethod
+    @plugins.HOOKIMPL
+    def beforeReactorConstruction(cs) -> None:
+        cs._beforeReactorConstructionFlag = True
+

 class UpInterface(interfaces.Interface):
     """
@@ -248,6 +254,7 @@ class TestUserPlugins(unittest.TestCase):
             TEST_ROOT, inputFileName="smallestTestReactor/armiRunSmallest.yaml"
         )
         fuels = r.core.getBlocks(Flags.FUEL)
+        self.assertTrue(o.cs._beforeReactorConstructionFlag)

         # prove that our plugin affects the core in the desired way
         heights = [float(f.p.height) for f in fuels]