Limited expected value, stop-loss and layer
severity.Rdlev(dist, limit) is E[min(X, limit)]; stop_loss(dist, retention) is
E[max(X - retention, 0)], computed directly so it stays accurate far into
the tail; layer(dist, limit, attachment) is the expected loss to the layer
limit xs attachment. Defined for lognormal and grid_distribution
severities and the Pareto family (pareto, piecewise_pareto,
log_affine_pareto, generalized_pareto).