Add option for rotated view of last marginalisation

[sebastian-schindler]

Add an option for displaying the last marginalisation (lower right 1D plot) with the x-axis vertically instead of horizontally. The rationale behind this is that the distribution matches the 2D plot better, especially in the case of only two dimensions: In this case and with such an option, corner would produce the typical plot of a 2D distribution in the center with marginalised distributions on the sides, like shown here.

To implement this it would be necessary to rotate the plot clockwise by 90 degrees (and potentially invert the x-axis?). This could be achieved with ax.hist(x, orientation='vertical').