Add plotting routine for creating "Manhattan" plots

[stufield]

• create a "Manhattan" plotting routine for package
• there is an existing routine from our internal code base, called plotManhattan() the could be used as a starting point.
• doesn't have to be identical, but could act as a basis
• we would have to figure out how to generate the effect sizes/p-values or (similar to plotVolcano()) have the user provide those as an argument, this is probably preferable, because calculating statistics is out of scope for this plotting package

[amanda-hi]

Our internal plotManhattan() uses a lot of other functions that will not be available to SomaPlotr, so I think I can use the ggplot2 code as a starting point, but will need to ditch all the other pre-processing steps. I'll have to start w/ SomaDataIO's example_data and re-create those steps, without using any internal (and currently publicly unavailable) functions (or rely on the user to do that part first, before creating the plot). Similarly to plotVolcano(), I think we'll need to let the user provide a lot more than the current plotManhattan()/plotVolcano() functions do, since we don't have access to all the other utility functions to do stats & data processing under the hood.

This is probably obvious to you already, but I'm just getting caught up and leaving myself some notes :) I'm starting on this now, but I'm going to need to figure out how I want to tackle this first.

[stufield]

Yes, I think you'll have to enter the workflow a little more downstream that the existing plot (which is fine, probably better) since you shouldn't really be doing any statistics "under the hood" of a plotting routine anyhow. So the input for the plot would be the output of the statistical analysis. This part we could show in the examples or vignettes. I think plotVolcano() already does this, or something similar, is that correct?