[WIP] Chemical Shift Wrappers

So I made a rough attempt at streamlining the calculation of chemical shifts via MDTraj.  There's still some work for me to do on futher standardizing the output style, checking for atom nomenclature changes, etc.  

Here's example driver script:

```
import pandas as pd
import mdtraj as md

trj = md.load("./trj.pdb")[0:3]

d0 = md.nmr.chemical_shifts_shiftx2(trj)
d1 = md.nmr.chemical_shifts_ppm(trj)
d2 = md.nmr.chemical_shifts_spartaplus(trj)

```

```
In [13]: d0[::150]
Out[13]: 
frame               0         1         2
resSeq name                              
41     C     175.1211  175.1211  175.1211
53     CB     34.1491   34.1491   34.1491
65     CD     29.1723   29.1723   29.1723
75     HB3     1.5067    1.5067    1.5067

[4 rows x 3 columns]

In [14]: d1[::150]
Out[14]: 
                  0       1       2
resSeq name                        
42     CA    55.044  55.044  55.044
72     CA    59.754  59.754  59.754

[2 rows x 3 columns]

In [15]: d2[::150]
Out[15]: 
frame              0        1        2
resSeq name                           
41     C     176.357  176.357  176.357
66     HN      8.003    8.003    8.003

[2 rows x 3 columns]
```