Determine 1D plot range from iso-probability points rather than tail percentiles

[lukashergt]

The way things are currently set up, the 1D plotting range by default is determined by a 5 sigma posterior interval, dropping the left and right percentiles.

This approach is rather bad for some setups, e.g. for a double Gaussian.

A better way would be not to calculate the percentiles from the left and right tails, but to determine the iso-probablility points for a given quantile and retain the leftmost and rightmost as plotting intervals.