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.