This is a method for the generic quantile
function for the class ewcdf of empirical weighted cumulative
distribution functions.
The quantile for a probability p is computed
as the right-continuous inverse of the cumulative
distribution function x (assuming type=1, the default).
If normalise=TRUE (the default),
the weighted cumulative function x is first normalised to
have total mass 1 so that it can be interpreted as a
cumulative probability distribution function.