[WIP] Clad Test

[ax3l]

• README: How to Set Up & Compile
• Minimal Example w/o external functions
• Differentiation need to be available to the AST, easiest way to achieve that is a single TU
  • A possible way for this is CMAKE_UNIT_BUILD. Works well for ImpactX, needs work for AMReX.
• reported error logs by email
• once we get further, I am not sure the if the MFIter logic in AMReX can be expressed in a single TU rn with AMReX not supporting unity builds yet... (and us generally, building an intermediate lib for AMReX). Once we get elements to build we could start with envelope tracking instead Track Covariance Map #814