sampleKappa: Sample a value from the Bessel exponential distribution
In circglmbayes: Bayesian Analysis of a Circular GLM

This is an implementation of the algorithm of Forbes and Mardia (2015) to sample from the Bessel exponential distribution, which is the conditional distribution of the concentration parameter of a von Mises distribution given the mean mu. The distribution is proportional to exp(- η g κ)/I_0(κ)^η. Note that beta_0 in Forbes and Mardia (2015) is renamed g here.

1	sampleKappa(etag, eta)

`etag`	Numeric; This is `eta * g`, which should `-R*cos(mu-theta_bar)`, where `R` is the posterior mean resultant length, and `theta_bar` is the posterior mean, while `mu` is the current value of the mean.
`eta`	Integer; This is the posterior sample size, which is n + c where c is the number of observations contained in the conjugate prior. For uninformative, `c = 0` and `eta = n`.