Melissa Spack Packaging

With the aim of simplifying Melissa installation procedure, several package managers are investigated. This MR focuses on packaging Melissa with Spack. It requires the following steps:

• removing all pip commands from cmake,
• building a CMake based melissa-api package,
• building a pip based py-melissa package with multiple variants,
• pushing the packages to Spack store (github),
• updating the doc (quick install, spack instal, advanced install, etc.).

Note: see this previous MR discussing packaging strategies and former Melissa spack packages.