Preliminary CMake implementation

This MR configures the mtime library with CMake.

• CMake setup

• Library configuration

• Test configuration

• Example configuration

• Documentation configuration

• [ ] Remove autotools config files

• [ ] CI works with CMake configuration

This MR should be merged after !16 (merged)

cmake/config.cmake.in

+# Copyright (c) 2013-2024 MPI-M, Luis Kornblueh, Rahul Sinha and DWD, Florian Prill. All rights reserved.
+#
+# SPDX-License-Identifier: BSD-3-Clause
+#
+
+@PACKAGE_INIT@
+
+include("${CMAKE_CURRENT_LIST_DIR}/@PROJECT_NAME@-targets.cmake")
+
+include(FindPackageHandleStandardArgs)
+find_package_handle_standard_args(@PROJECT_NAME@ REQUIRED_VARS @PROJECT_NAME@_DIR)