y-axis gene tracks missing from trans heatmaps

[Thomas Gilgenast]

when using add_refgene_stack() or add_refgene_stacks() on an ExtendableHeatmap whose grange_x and grange_y are on different chromosomes, reference genes never appear on the y-axis

this is due to a bug in add_refgene_stack() which causes it to use grange_x['chrom'] to decide what chromosome to load regardless of the track orientation

[Thomas Gilgenast]

fixes #69

→ <<cset f3f15ce81a7f>>

[Thomas Gilgenast]

there’s a demo showing this fix in action at https://colab.research.google.com/drive/1z355z5PRnY3GpwmH_mbMHaqCdGQc4nQo