Add LMfit-based Gaussian and Voigt fitters

[joefowler]

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

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I might have missed it, but the legacy GaussianFitter and VoigtFitter seem to be the only ways to fit a single Gaussian or Voigt in Mass. We want LMfit-based ways to do this that are similar to the new GenericLineModel.

[joefowler]

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

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@{557058:9524f6a0-ef9f-4466-b913-d04e51ff9093} points me to quick_monochromatic_line which might meet this need. If so, then update the file LineFitting.md to explain how to use it for Voigt, Lorentz, and/or Gaussian fitting, and no new code will be required.

[joefowler]

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

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Add demo of fitting a Gauss, Lorentz, or Voigt function

Fixes #201. Test output may not be correct yet, though.

[joefowler]

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

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Removing milestone: v0.7 (automated comment)