initial commit for adding descriptor of water features

[msultan]

describe water features. Output after using pandas is something like this.

In [24]: df_w[:1]
Out[24]: 
  atomind      bigclass otherInfo resSeq resid resname  smallclass
0     [0]  water_shells      None    [1]   [0]   [LYS]           0

In [25]: df_w[-1:]
Out[25]: 
     atomind      bigclass otherInfo resSeq  resid resname  smallclass
1487  [4247]  water_shells      None  [263]  [262]   [TYR]           1

[msultan]

 from wetmsm import SolventShellsFeaturizer

 import pandas as pd
 import mdtraj as mdt

 trj  =  mdt.load("prot.pdb")

  solute_ind = [i.index for i in trj.topology.atoms if \
  (i.element.name=="nitrogen" or  i.element.name=="oxygen") and i.residue.is_protein]
 solvent_ind = [i.index for i in trj.topology.atoms if \
 (i.element.name=="oxygen" and i.residue.is_water)]

  w_feat = SolventShellsFeaturizer(solute_ind,solvent_ind,2,0.3)
  df_w = pd.DataFrame(w_feat.describe_features(trj))