Add correct legend support for real number plots <3

Previously, real numbers legends were not great. The raster function
treats them as integers. This is fine for numbers larger than say 3,
where integer representation for the colour palette is ok. Here, numbers
<=3 are now treated as real numbers in legends, so decimal places occur,
and continuous colours occur.
adj - adding drop=FALSE matches

89783bcdf71a55e99999f3da90017eb90ebcae0a
Path for data in the shiny_succession server.R
Plot - improve colours & remove extra spaces after commas
minor to fireSpread in sampleModules