Improved multipole acceptance criterion (MAC)

Big improvements to the gravity code.

Improvements:

• Fix mistakes in the higher-order derivatives used below softening,
• Implement the Dehnen+2014 multipole acceptance criterion (in cartesian coordinates),
• Allow (optionally) to use the tree below softening,
• Allow (optionally) to use the truncated and softened potential in the MAC,
• Fully simplify the first order multipole terms that are 0,
• Simplify the signature of the long-range and short-range gravity truncation functions,
• Do not use the M2P kernel if the multipole is made of one single particle,
• Vectorize the cache construction and write backs to memory,
• Vectorize the M2P kernel,
• Use a leaf-level cell lock rather than the field-by-field atomic operations on the particles,
• Significant update to the gravity theory doc.

Still to do:

• Correctly use and document the new truncated MAC,
• Re-implement the MAC for the proxies,
• Update all the example YAML files,
• Clean-up the debugging checks,
• Prevent M2L for cells with small number of particles.