Split function_space_impls.py into smaller files

The file function_space_impls.py has reached a length of 1053 lines. Adding support for additional Finite Element spaces will make it grow further.

The primary purpose of this MR is to split it up into several files:

• one for the abstract base class FunctionSpaceImpl
• one for each Finite Element space (here we combine scalar and vectorial Langrage spaces into one file, e.g. p2_space_impl.py)
• one file for the factory function create_impl() which becomes a free-function instead of a static class method to avoid problems with cyclic dependencies

The files are collected inside their own folder function_space_implementations inside operator_generation.

The secondary purpose is to adapt the code in OperatorGenerationTest.hpp to a name change in HyTeG, see hyteg!849 (merged) and hyteg!851 (merged).

hog/operator_generation/function_space_implementations/__init__.py

+# HyTeG Operator Generator
+# Copyright (C) 2024  HyTeG Team
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+__all__ = [
+    "function_space_impl_base",
+    "function_space_impl_factory",
+    "p0_space_impl",
+    "p1_space_impl",
+    "p2_space_impl",
+    "n1e1_space_impl",
+]