Clean up the repository

A minor clean-up of the repository:

• remove the seemingly stale NEWS file;
• remove the seemingly obsolete pfunit directory;
• make the list of authors a markdown file for consistency and reference it in README.md;
• fix up REUSE.toml.

pfunit/tests/GNUmakefile

-# Copyright (c) 2013-2024 MPI-M, Luis Kornblueh, Rahul Sinha and DWD, Florian Prill. All rights reserved.
-#
-# SPDX-License-Identifier: BSD-3-Clause
-#
-
-.PHONY: clean tests
-
-SRCS = $(wildcard *.pf)
-OBJS = $(SRCS:.pf=$(OBJ_EXT))
-
-tests: $(OBJS)
-
-testSuites.inc: $(SRCS)
-
-FFLAGS += -I$(SRC_DIR) -I$(PFUNIT)/mod
-
-%.F90: %.pf
-	$(PFUNIT)/bin/pFUnitParser.py $<  $@
-
-%$(OBJ_EXT): %.F90
-	$(F90) -c $(FFLAGS) $<
-
-clean:
-	$(RM) *.o *.mod