plot_frequencies_heatmap is in snp_frq.py when it is used to plot all kinds of frequencies heat maps. It could probably be moved to util.py (if there are parasite functions that want to use a similar function) or anopheles.py.

This issue replaces the second part of #661.

Here is @leehart comment on this issue from #661:

I'm less comfortable with migrating plot_frequencies_heatmap to util.py or anopheles.py. I'm wary about files like util.py becoming a general purpose catch-all place for all kinds of stuff, like that stereotypical one kitchen drawer! And if we're talking about moving stuff out of anopheles.py, which sounds like a pretty high-level place, something seems wrong about moving other stuff into it, especially plotting functions, within the same breath. But I would need to refresh my memory on the scope of those particular modules. In principle, I would go with grouping like-with-like, but you know those sort of decisions can become subjective or aesthetic, rather than perfectly rational.

Where do we usually put functions (e.g. plotting functions) that are used by both parasite and vector? Is it really util.py?