Remove GSL dependency

Hi this PR is removing GSL dependency.

It appears that the GSL dependency is used for two differents things in neBEM :

1. Calculate complete elliptic integral E(k) and K(k) but in c++17 this functions are available in the STL
2. Do some matrix inversion, however there are 2 others options and the interfce only allows the two others. Then GSL dependency can be removed. It will helps a lot if we want to make it compiles in Windows.

This PR remove some dead code in neBEM too. It could maybe solve the problem of linking in macOS as some function where declared but not defined !

For this ROOT would be better to be compiled with c++17 so CI has to be performed at least in fedora 34 (ROOT 6.22.08) which is very close to the actual fedora 32 root version (6.22.02).