Lambda-CCSD

[lelemmen]

Short description

This PR will gather work related to the implementation of the Lambda-CCSD equations and CC 1- and 2-DMs.

To do

• (if applicable, when creating new source files): don't forget to update the Doxygen file, the collective gqcp.hpp include header and the CMake files
• Add other smaller task to be completed as a Markdown task list