remove dependency on gcammaptools

[abigailsnyder]

This consistently causes issues in installations and testing. As far as I can tell, we only use a call to gcammaptools in the plot_field function in fldgen/R/plot.R and then in calls to plot_field from the vignettes. This is a handy convenience function that makes pretty maps, but in no way core to the science we're trying to do.

• [x] @kdorheim track down and comment out calls to plot_field. And comment out most of the interior of plot_field as well. Hopefully not too much to track down and then that lets us move on to the next issue with setting up testing.

• [x] @abigailsnyder implement a different interior of plot_field that doesn't require call to gcammaptools. Will probably sacrifice having a nice map projection in the short-term at least. Just do a quick rectangular map plotting so that we have a convenience function that works and we can call from vignettes. We can improve the aesthetics of its maps down the road or not (a user could obviously make their own map if they really hated ours).

[abigailsnyder]

addressed with https://github.com/JGCRI/fldgen/pull/46