Add explicit GSL support

This MR adds explicit support for the GSL library in the radler CMakeLists.txt. This solution solves a build issue on various systems (CSCS, EPFL ...).

When installing radler as a standalone library (not building WSClean), the CMakeLists.txt finds GSL:

-- Found GSL: /users/scorda/spack940/opt/spack/cray-cnl7-haswell/gcc-9.3.0/gsl-2.7.1-fbkto3urzy3kepm5flhvkgy4ek3uler3/include (found version “2.7.1"

However, during the build phase it fails with the following issue:

[ 56%] Building CXX object cpp/CMakeFiles/radler.dir/algorithms/ls_deconvolution.cc.o
/users/scorda/radler/cpp/algorithms/ls_deconvolution.cc:5:10: fatal error: gsl/gsl_vector.h: No such file or directory
 #include <gsl/gsl_vector.h>
          ^~~~~~~~~~~~~~~~~~
compilation terminated.
make[2]: *** [cpp/CMakeFiles/radler.dir/build.make:160: cpp/CMakeFiles/radler.dir/algorithms/ls_deconvolution.cc.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:231: cpp/CMakeFiles/radler.dir/all] Error 2
make: *** [Makefile:136: all] Error 2

This problem does not arise in DAS-6, because CMake finds the GSL header in /usr/include/gsl/.