Melissa - Spack install

This MR aims at adding an installation procedure based on Spack.

The original Melissa version was already supporting this alternative:

• melissa-launcher build
• melissa build
• melissa-da build

Note: Explanations on how to build a Spack package are available in this tutorial and this guide

With the latest version of Melissa, significant changes were made to the installation procedure which currently mixes CMake and pip instructions. Supporting Spack packaging logic will require the following adjustments:

• purging the main CMakeLists.txt from all unnecessary variants,
• reducing .local installation to common python packages,
• installing melissa-launcher and melissa-server in /path/to/melissa/package/in/spack/bin,
• identifying the minimal Python version.