Add `MatrixRef` to take a sub-matrix from an existing `Matrix` #3906

	#
	# Distributed Linear Algebra with Future (DLAF)
	#
	# Copyright (c) 2018-2023, ETH Zurich
	# All rights reserved.
	#
	# Please, refer to the LICENSE file in the root directory.
	# SPDX-License-Identifier: BSD-3-Clause
	#

	name: Check format

	on:
	- pull_request

	jobs:
	check:
	runs-on: ubuntu-22.04

	steps:
	- uses: actions/checkout@v3
	- uses: actions/javascript-action@v1

	- name: Install tools
	run: \|
	sudo apt-get update
	sudo apt-get install --no-install-recommends clang-format-15 python3
	pip3 install black==23.03.0
	pip3 install cmakelang==0.6.13

	- name: Fetch master
	run: git fetch --no-tags --depth=1 origin +refs/heads/master:refs/heads/master

	# Fails if there are tabs in source files.
	- name: Check formatting
	run: ./.github/format.sh

	- name: Check scripts formatting
	if: always()
	run: black --line-length 105 scripts/

	# Fails if there are differences.
	- name: Formatting issues
	if: always()
	run: git diff --color --exit-code

	- name: Check include guards
	if: always()
	run: \|
	find include test -type f '(' -name ".h" -o -name ".h.in" ')' \
	\| xargs -I{} sh -c \
	"egrep -sq '^#pragma once' {} \|\| echo {}" \
	> pragma-once.check

	for filepath in `cat pragma-once.check`; do \
	echo "::error file=$filepath,line=1::missing include guard in $filepath"; \
	done

	test ! -s pragma-once.check

Provide feedback