Avoid error in R >= 4.3.0

[ranghetti]

Using R 4.3.0 an error occurs due to the first user-visible change reported in the R 4.3.0 changelog; this PR fixes that. Note that the new syntax should keep the original output value, but I have not performed any test on it.

Moreover, I edited the DESCRIPTION file in order to install RStoolbox from GitHub repo, since it was removed from CRAN (maybe you could also consider to remove this dependence editing the code where ggR() and ggRGB() are used).

Thank you