Determine the full width at half max of a curve.

The full width at half maximum (FWHM) is a parameter which is used for describing the width of a peak. https://en.wikipedia.org/wiki/Full_width_at_half_maximum

It is defined as the distance between the two points at which the curve reaches half of its maximum value. For example if y = [0 3 6 3 0], the full width at half of y is 2.

The data in the test-set have only one positive peak. The test suite checks if the solution is correct within an error of 10%. Note that the input array may not necessarily contain an element equal to half of the maximum.