Fit: Add SNIP background subtraction

This MR adds a background subtraction using snip (http://www.silx.org/doc/silx/0.9.0/modules/math/fit/filters.html#silx.math.fit.snip1d; DOIs of references are available: https://github.com/silx-kit/silx/pull/2240/files#diff-3e7d79768739060243e7d7bf1c1a3184R46). This ensures the background is always below the profile (which is not the case with the linear background from first to last data point).