Molecule Polymer Comb
=======================
Arguments:

  * `nmolecule` (int) - Number of molecules in simulation
  * `moleculeX` (json object) - Information about `moleculeX` where `X` ranges from 0 to `nmolecule-1`. In each `moleculeX` object the following must be specified:

    * `type` (string) - Type of molecule. Here `PolymerComb`
    * `label` (string) - label for this molecule, defaults to `molX` 
    * `chain_type` (string) - Type of chain statistics. Either `discrete` (as documented here) or `continuous`.
    * `volume_frac` (float) - Volume fraction of `moleculeX` in the simulation. Must be 0-1.
    * `backbone` (json object or dict) - Backbone chain information. Required arguments depend on `chain_type` specified earlier. For example, if `chain_type=discrete` the arguments `nbeads`, `nblocks`, `block_species`, `block_fractions` (or `monomer_sequence`) must be specified. See :ref:`Molecule Polymer Linear (continuous)` and :ref:`Molecule Polymer Linear (discrete)` for required arguments.
    * `sidechainX` (json object) - Sidechain information of `sidechainX` where `X` starts at 1. In each `sidechainX` object, all information of the corresponding `chain_type` must be specified (see `backbone` argument above). Additionally, the following argument must also be specified:

        * `graft_locations` (list int) - the indicies of backbone sites that this sidechain will be grafted to. Indexing starts at 0.
      

Example (python)::

  # this creates an A-B bottlebrush
  fA = 0.5  # fraction of A beads
  N_BB = 50 # backbone length
  N_SC = 10 # sidechain length
  B_graft_start = int(fA*N_BB)

  fts.add_molecule(type='PolymerComb',\
      chain_type='discrete',\
      backbone=dict(nbeads=N_BB,nblocks=2,block_fractions=[fA, 1-fA],block_species=['A', 'B']),\
      sidechain1=dict(nbeads=N_SC,nblocks=1,block_fractions=[1.0],block_species=['A'], graft_locations=list(range(0,B_graft_start))),\
      sidechain2=dict(nbeads=N_SC,nblocks=1,block_fractions=[1.0],block_species=['B'], graft_locations=list(range(B_graft_start,N_BB))),\
      volume_frac=1.0)

Example (json)::

  "molecules": {
    "molecule0": {
      "backbone": {
        "block_fractions": [0.5,0.5],
        "block_species": ["A","B"],
        "nbeads": 50,
        "nblocks": 2
      },
      "chain_type": "discrete",
      "sidechain1": {
        "block_fractions": [1.0],
        "block_species": ["A"],
        "graft_locations": [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24], 
        "nbeads": 10,
        "nblocks": 1
      },
      "sidechain2": {
        "block_fractions": [1.0],
        "block_species": ["B"],
        "graft_locations": [25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49], 
        "nbeads": 10,
        "nblocks": 1
      },
      "type": "PolymerComb",
      "volume_frac": 1.0
    },
    "nmolecule": 1
  },



Molecule Polymer Comb Formalism
--------------------------------------

This is described in Levi, Lequieu et al. Macromolecules (2020). Eventually should sumarize here.