unisliceL: Slice Sampler
In BayesBD: Bayesian Inference for Image Boundaries

Performs slice sampling for lambda parameter. This is a helper function used by BayesBDbinary and BayesBDnormal.

1	unisliceL(x0, gx0, i_J, tauini, anini, alphalambda, betalambda, lambdaini)

`x0`	initial value of lambda.
`gx0`	likelihood at x0.
`i_J`	number of basis functions (2*i_J+1).
`tauini`	tau parameter in covariance function.
`anini`	2J+1 vector of eigenfunction coefficients.
`alphalambda`	hyperparameter for sampling tau, usually 1.
`betalambda`	hyperparameter for sampling tau, usually 1.
`lambdaini`	current value of lambda.