Support for multiple softening lengths in the gravity solver

Implements the following:

• Add background DM particles (type 2).
• Different softening lengths for DM, baryon and background particles.
• Background DM particles have a softening length that scales with mass^(1/3).
• Two particles interacting directly use the largest of the two softenings.
• The multipoles carry the maximal softening length of any particle within the multipole
• The multipole-multipole or particle-multipole calculations are only allowed if the distance is larger than the softening.
• Fixes #599 (closed).
• Update all the examples to use the new names for the softening lengths in the YAML files.
• Update the RTD description of the Gravity parameters and i/o.
• When replicating the ICs, replicate the black hole particles. Prevent replication when running with background DM.