Change handling of exponential tails

Original report by Joseph Fowler (Bitbucket: joe_fowler, ).

Currently, we allow lines to have exponential tails with fractional weight of up to 100% on both the low and high-energy sides of the main Gaussian. But when their sum exceeds 100%, this makes no sense.

Proposal: change the meaning of the parameters, and change one name to hint at the new distinction.

Old way

tail_frac was the fraction of all events that end up in the low-energy tail. Allowed values: [0,1]
tail_frac_hi was the fraction of all events that end up in the high-energy tail. Allowed values: [0,1]

New way

tail_frac is the fraction of all events that end up in either tail. Allowed values: [0,1]
tail_share_hi is the fraction of all tail events that end up in the high-energy tail. Allowed values: [0,1]

The use of “share” instead of its approximate synonym “frac” is deliberate: it hints at the fact that they are semantically different things, even if admittedly the 2 words are kind of synonyms.

It’s possible that this is related to the new problem with errors found in scipy/lmfit (issue #251). But even if it’s not, we should make it impossible for the fitter to explore unphysical values of tail-related parameters and make the proposed change.

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usnistgov / mass

Change handling of exponential tails #252