Rely on SaturationTransformation internals

[wd60622]

The _plot_response_curve_fit function in the MMM class can rely on the sample_curve method of the SaturationTransformation class.

From there, the quantiles can all be calculated at once instead of by channel. The current limitation of the sample_curve is that all of the
x values are in their scaled form, so the channel loop would have to scale back the x values to their original form.

pymc-marketing/pymc_marketing/mmm/mmm.py

Lines 1669 to 1678 in f70db33

	y_fit = self.saturation.function(
	x=x_fit, **mid_params[channel]["saturation_params"]
	).eval()
	y_fit_lower = self.saturation.function(
	x=x_fit, **lower_params[channel]["saturation_params"]
	).eval()
	y_fit_upper = self.saturation.function(
	x=x_fit, **upper_params[channel]["saturation_params"]
	).eval()