adding an inverse gamma class

I've been using GaussianFixedMean to effectively implement an Inverse Gamma prior on the covariance of a scalar Gaussian, but it seems nicer to separate this functionality into a separate class. It seems more natural to me at least.

The main issue is that resample, as I've coded it, requires `data` to be an array of Gaussian residuals, which differs from the convention you've used so far. Feel free to reject or suggest alternatives.