implot ignoring gridcolor argument if cmap is defined

[astro-jcmcg]

I've been making some plots and noticed that the gridcolor argument for implot only seems to function for the default colour map. In this case it seems like the grid lines end up the same colour as the lower extreme of the colour map.

[sefffal]

Thanks for the bug report @astro-jcmcg!

[icweaver]

We're investigating switching to Makie.jl, which will hopefully solve issue like this out-of-the-box