Group names extracted from filenames

Short fix for proper label on plots for group names extrated from filenames (with substr function). It is done with factor function that labels the levels of factors.

explosive<-substr(files, 1, 4)
explosive<-factor(explosive, levels=c ("C_4_", "PETN", "RDX_", "TNT_"), labels=c ("C-4", "PETN","RDX", "TNT"))
d1 = data.frame(files, explosive)
#create hyperSpec object
a1= new ("hyperSpec", wavelength = wavenumber, spc = spc, data=d1)
a1