add option to plot only longest isoform

[Thomas Gilgenast]

when plotting heatmaps of wide zoom windows, sometimes the large number of isoforms for one gene will dominate the plot area, leaving very little space for the actual heatmap - in these situations it may be desirable to plot only one isoform from each gene, and then potentially re-draw the plot with a smaller zoom window if we need to look at every individual isoform later

in GeneExtendableHeatmap, the class methods add_refgene_stack() and add_refgene_stacks() should accept a all_isoforms=True kwarg

when False is passed, the genes should be grouped by name, and for each name only the longest gene with that name should be included in the final list passed to add_gene_stack()

[Thomas Gilgenast]

another possible way to address the core issue is to force all genes to be plotted in one row, skipping the packing/stacking and adding only one gene track per axis