Use energy proxy mapping from TXS 0506+056 data release to build energy PDfs

[robertdstein]

Is your feature request related to a problem? Please describe.
Currently, it is not possible to convert true energy into reconstructed energy proxy for public data.

Describe the solution you'd like
A mapping between true energy vs energy proxy was recently published by IceCube (Oct 22nd, 2019 at https://icecube.wisc.edu/science/data/TXS0506_point_source). This can be incorporated into the code.

Describe alternatives you've considered
Assuming energy proxy = lognormal centered on true energy, or exactly equals true energy. But that's dumb/oversimplified, and does not recreate PDFs properly.

[robertdstein]

Somewhat overtaken by #39 with the new data release, but the basic principle still stands