Fix heatmap

[yihming]

Consider 3 cases on showing groupby tick labels:

• When show_sample_name=True, set yticklabels to sample_tick_labels;
• When on_average=True, set yticklabels to cluster_ids;
• When on_average=False, set yticklabels to [].

Tick label rotation:

• Rename attrs_labelrotation to xticklabel_rotation, and set its default to 90.0.
• Remove groupby_labelrotation as y tick labels don't need rotation.

No need to apply remove_unused_categories(), since pandas df.groupby(..., observed=True) would cover it.