chromplotter.ipynb produces non-square matrix plots

[dmalzl]

The chromplotter.ipynb notebook is intended to enable the plotting of whole chromosome matrices either alone or in comparison. These matrices should be square, however, for some matplotlib versions the plot_whole_chrom_contact_matrix generates non-square heatmaps due to the tickpos setting invoking a reset of the y axis limits. This can be addressed by simply putting ax.set_ylim(xmax, xmin) after setting the ytick positions.