Optimization algorithm to determine HDIs does not converge well

[michaelosthege]

While working on #12 I added tests and came up with one to check if calibr8.core._get_hdi is doing a good job.

It does not.

I believe the scipy.optimize.fmin doesn't cope well with the switching inside the hdi_objective, or at least not in combination with the step-function like area calculation by _interval_prob.

Some starting points to fix this:

• Make a detailed heatmap visualization of the objective function that's being optimized, based on a coarse x_cdf and cdf input. (Hypothesis: It's not smooth, but has step functions in one dimension.)
• Adapt test_interval_prob to an interpolation that should give much more accurate results compared to the step-function. (Also nicer conceptually!)
• Use the triangular example from test_get_hdi as a starting point, because it's HDI is easy to determine analytically (centerline of the triangle to its right flank).