Improve plotting for learners

Currently all plots (and live plots) are done using holoviews. There are some points where this could be improved.

• The 2D learner (and possibly the 1D leaner too) works with functions that return a vector of values. However when calling the plot method it will only plot (and live plot) the first element of this array even though all are present in learner.data
• There is no colorbar to quantify what z-value that was extracted.
• There are no sensible labels and units added to the axes, it may be possible to optionally specify this to the learners.

Below is an image of a leaner that

• uses a function that returns a vector
• should have labels and units on the x-axis
• has no colorbar