Develop PlotEquiMap with degree symbol in lons and lats labels

Hi @aho

I hope to be on time to add this feature to PlotEquiMap for s2dv release.

Please, consider the code in this branch and the following example:

data <- CSTools::lonlat_data$exp
source("/esarchive/scratch/nperez/git/s2dv/R/ColorBar.R")
source("/esarchive/scratch/nperez/git/s2dv/R/clim.palette.R")
source("/esarchive/scratch/nperez/git/s2dv/R/Utils.R")
library(maps)
PlotEquiMap(data$data[1,1,1,1,,], lon = data$lon, lat = data$lat, degree_sym = TRUE)

Which result is:

Thanks,

Núria