Gaussian guess - guess negative Gaussian if peak 'looks' negative.

If average([y value at minimum x], [y value at maximum x]) > (min(y) + max(y))/2, guess a negative Gaussian instead of a positive one.

Acceptance criteria

• For a Guassian with tails higher than midpoint of y-values, negative Gaussian is guessed
• For a Gaussian with tails lower than midpoint of y-values, positive Gaussian is guessed

Notes

• Relevant code is here: https://github.com/ISISComputingGroup/ibex_bluesky_core/blob/5de624a00d4236d36f0d2e606afea9c22892e842/src/ibex_bluesky_core/fitting.py#L159
• When guessing a negative Gaussian:
  • We still want to guess the centre & width using centre-of-mass - but this will now be centre-of-mass of inverted data. The easiest way to do this will be to guess the centre & width using cen, width = _guess_cen_and_width(x, -y).
  • Background guess is max(y)
  • Amplitude guess is min(y) - max(y)