Use logA instead of As when sampling / minimizing

[andreufont]

As has values of order 1e-9, while other pararameters are order unity. This might make life uneccessarily difficult to the sampler / minimizer.

[andreufont]

Of course, that is not correct. We already use "unit cube" parameters in the likelihood, so we can probably delete this issue.

[andreufont]

Leaving it open, since it might be useful for plotting.

[andreufont]

I was hit again by this, when trying to compute Gaussian contours (ellipses) around the best fit parameters. Having the x and y axis differ by 9 orders of magnitude is a numerical pain. I decided to plot 1e9 * As, probably good for now.