Change safe energy header keywords

[cdeil]

We are currently using the LO_THRES and HI_THRES keywords for the safe energy thresholds in the ARF and AEFF formats: http://gamma-astro-data-formats.readthedocs.io/en/latest/search.html?q=LO_THRES&check_keywords=yes&area=default

I saw that in this OGIP spec, LO_THRES means something different: http://heasarc.gsfc.nasa.gov/docs/heasarc/caldb/docs/memos/cal_gen_92_002/cal_gen_92_002.html#tth_sEc4 In the RMF it's

LO_THRES - minimum probability threshold used to construct the matrix (matrix elements below this value are considered to zero and are not stored)

Given that it's a cryptic term anyways, I'd suggest we change to something else. E.g. EMINSAFE, EMAXSAFE or really anything else that doesn't conflict with the exiting usage and maybe suggest energy range from the name.

@jknodlseder @cboisson @registerrier @joleroi - Thoughts?