Extend data support add more sources and selection of effects

[samaloney]

Espically for higher energy ranges (tens of MeV) the current data source end at 20 MeV but other data sources go well in to MeV range:

• NIST FAST
  • f1, f2, Photoelectric, Coh+Inc, Total
• NIST XCOM
  • Total Attenuation with Coherent Scattering, Total Attenuation without Coherent Scattering, Coherent Scattering, Incoherent Scattering, Photoelectric Absorption, Pair Production in Nuclear Field, Pair Production in Electron Field

Not all sources provide exactly the same data so it would be nice to have a higher level interface also the capably to turn on/off various effects e.g. coherent v incoherent would be useful. As the data seem to be hidden behind we UI could write some code to scrape the data and include with release. The user could then run the same code to get updated versions

[samaloney]

See comments in #24 too

[ehsteve]

This is a great idea!