plot_coverage: Plot hex coverage for a given eccentricity
In mrc-ide/RMAPI: Mapping Averaged Pairwise Information in R

Plot hex coverage (number of intersecting ellipses) for a given eccentricity.

1	plot_coverage(proj, eccentricity = 0.9, n_ell = 20, return_type = 1)

`proj`	object of class `rmapi_project`.
`eccentricity`	eccentricity of ellipses, defined as half the distance between foci divided by the semi-major axis. We can say e = sqrt{1 - b^2/a^2}, where e is the eccentricity, a is the length of the semi-major axis, and b is the length of the semi-minor axis. Eccentricity ranges between 0 (perfect circle) and 1 (straight line between foci).
`n_ell`	number of points that make up an ellipse.
`return_type`	format of return object: 1 = a `gridExtra` object, 2 = a list of `ggplot2` objects.