Cmake and guix

[q-posev]

Additions:

1. CMake build: CMakeLists.txt files in the root, src and tests directories. Builds, tests (using CTest), and installs TREXIO both in developer and user mode. All necessary files are propagated to the distribution produced by make dist (with Autotools) so that the user can build TREXIO distribution using CMake;
2. FindTREXIO.cmake module in cmake directory. This file has to be included by the CMake projects using TREXIO as a dependency. Required to enable find_package(TREXIO) in CMake;
3. Preliminary trexio.scm schema file in tools directory. This file is required for the guix package manager;
4. Minor changes related to Autotools (e.g. introduced AC_CONFIG_AUX_DIR).