[ENH] Add atomic scattering factors

[lmmentel]

Add new model ScatteringFactor with atomic scattering factors from this data source suggested by @goodwin2208 in #173.

Elements now have an additional property named scattering_factors that returns a list of ScatteringFactor instances for that element. For example:

>>> from mendeleev import Fe
>>> Fe.scattering_factors
[Z=26 E=10.0 f1=-9999.0 f2=1.37852,
 Z=26 E=10.1617 f1=-9999.0 f2=1.42961,
 Z=26 E=10.3261 f1=-9999.0 f2=1.48259,
 Z=26 E=10.4931 f1=-9999.0 f2=1.53754,
 Z=26 E=10.6628 f1=-9999.0 f2=1.59453,
 Z=26 E=10.8353 f1=-9999.0 f2=1.65362,
 Z=26 E=11.0106 f1=-9999.0 f2=1.71491,
 Z=26 E=11.1886 f1=-9999.0 f2=1.77847,
 Z=26 E=11.3696 f1=-9999.0 f2=1.84438,
...

ScatteringFactor has following attributes:

• atomic_number
• energy
• f1
• f2

closes #173