Add rgsl

This will add the rgsl package (this is one of ours), with the library dependency on GSL (GNU Scientific Library).

packages/r-rgsl/package.py

+from spack.package import *
+
+class RRgsl(RPackage):
+    """This is an interface package to solve ODE systems using gsl_odeiv2 within R. The system to solve has to be defined using C code: a shared library, dynamically loaded by this package when solving."""
+
+    homepage = "https://github.com/icpm-kth/rgsl"
+    url = "https://github.com/icpm-kth/rgsl/archive/refs/tags/v0.1.tar.gz"
+    version("0.1", sha256="dd6a14dda8731f4cf32149c8a9842e4e2ba424c8404f4d6839108153b20e4c3e")
+
+    # GNU Scientific Library: https://www.gnu.org/software/gsl
+    # spack: https://spack.readthedocs.io/en/latest/package_list.html#gsl
+    depends_on("gsl")