Upd readme

Context

Following some issues (like #241 (closed)) reporting installation does not work anymore simply doing pip install . in the main repository, the README.md needed to be updated along other installation files.

• README.md
  • update installation instructions
  • new file structure
  • add the Aidge logo
• setup.sh
  • Change help display method
  • Treat unset variables as an error when substituting
  • Dynamically select default compiler with OS
  • Dynamically select default number of jobs with OS
  • Add logging informations
  • Add aidge install path selection with '--prefix'
  • Add explicit c and cxx compilers selection '--cc' and '--cxx'
  • Order sections of the script in callable functions
  • add CMake compilation optimization option if MacOS detected
  • enhancement: allow for script parameters to be passed down to setup.py files by creating environment variables