ENDFtk v0.5.0

[whaeck]

This PR is for the v0.5.0 release which adds the following:

• covariance data in MF31, MF32, MF35 and MF40
• the new MF7 MT451 section for general information about thermal scattering
• adding a few convenience constructors in MF3, MF9 and MF10 sections
• adding convenience has_section(mf,mt) functions to the ENDF tree components

[whaeck]

Alea iacta est: it's been a long time coming ...