Remove short-lived heap allocations of strings in LArG4Barrel

As mentioned in	ATLASSIM-2643, lines using char* literals cause	short-lived
heap-allocations, harming simulation performance. This merge request aims
to mitigate this by making the strings in question static const.


Former-commit-id: 9680dee1dc3df5ad5ad7958a1fda116ddf76d85f