Add IsotopeDecayMode model and data

[lmmentel]

Add new model for decay modes of isotopes and populate with data from NUBASE

In [1]: from mendeleev.models import Isotope, IsotopeDecayMode
In [2]: from mendeleev.db import get_session
In [3]: s = get_session()
In [4]: he = s.query(Isotope).filter_by(atomic_number=2, mass_number=6).one()
In [5]: he.decay_modes
Out[5]: 
[<IsotopeDecayMode(id=11, isotope_id=14, mode=B-, intensity=100.0)>,
 <IsotopeDecayMode(id=12, isotope_id=14, mode=B-d, intensity=0.000278)>]

Added new function for accessing isotopes by atomic and mass number or symbol and mass number

from mendeleev import isotope
isotope("Li", 11)

TODO:

• documentation
• tests