PXL 4.0.

• Big refactoring of the class structure.
• Removed astro package.
• Disabled python and scripting packages.
• Update IO (breaks old file format as agreed upon).
• Use variants for user records.
• Use variants for module options.
• Store variant maps as AssocVectors.
• Switched from XML to JSON format for saving analyses.
• Build docs with breathe and sphinx.
• Added full CI setup.
• Added docker images.

Also, the tag naming scheme is changed to v<major>.<minor>.<patch> from now on!