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.

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added 27 commits

• 0d1c2ebb...8c86c97b - 25 commits from branch master
• 66f9ca53 - Merge branch 'master' into multi_softening
• 94171cb3 - Make the EAGLE BH model use the new softening definitions for the repositioning.

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assigned to @matthieu

@rttw52 could you check whether this version still works on your SIBELIUS zoom test? I am interested in the SWIFT+VR run and check whether the contamination is still not a problem in the high-res. region.

@jborrow could you check whether the nIFTy still behaves as you expect now that gravity interactions are correctly computed for particle of different species?

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• 559623a8 - Simplify the gravity potential calculations for M2L and M2P since the softened…

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• 4d62e1b1 - Only read the DM softenings if we have DM particles. Same fr baryons.

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marked as a Work In Progress

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• 49d09869 - Extended error message in M2P.

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• d4a014d1 - Avoid false-positive debugging check triggering because of rounding error.

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• b6aea014 - Avoid false-positive debugging check triggering because of rounding error.

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• bd40b3cc - Better error message in the part linking check.

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added 59 commits

• bd40b3cc...fa8aea3e - 57 commits from branch master
• 28f74883 - Merge branch 'master' into multi_softening
• e6501e1a - Make the multi-softening gravity code also compute the potential.

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• 97919494 - Adapt the other gravity schemes to use the new softening variable names.

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• c6b93b8d - Make the multi-softening scheme the default gravity scheme.

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unmarked as a Work In Progress

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• 6add4972 - Make the VR interface recover the mass of the first high-resolution DM particle…

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added 28 commits

• 6add4972...2ae2ec49 - 25 commits from branch master
• 3a9b39b7 - Merge branch 'master' into multi_softening
• 36046826 - Updated all the EAGLE examples to use the new softening names in the YAML files.
• e50450ab - Updated all the other examples to also use the new softening definitions.

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• 0e297e75 - Simplify the calls to potential_derivatives_compute_M2P() and…
• ffffbcd6 - Doxygen fixes.

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• 632c6d1e - Added description of the softening lengths to the parameter file RTD.
• d4fd4da1 - Added description of the PartType2 in the ICs and snapshots.

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