Update CLI help and update CHANGELOG

CHANGELOG.md

@@ -14,6 +14,7 @@ it in future.
 
 ### Added
 
+* Add option for helium-filled decay volume
 * Add pre-commit config: This will be phased in to improve code quality and spot issues as early as possible. Status visible in README and CI enabled for new pull requests.
 * Add CHANGELOG.md
 * Add `.git-blame-ignore-revs` to allow automatic reformatting etc. without polluting git blame

macro/run_simScript.py

@@ -138,7 +138,13 @@
 parser.add_argument("--scName", help="The name of the SC shield in the database", default="sc_v6")
 parser.add_argument("--MesonMother",   dest="MM",  help="Choose DP production meson source", required=False,  default=True)
 parser.add_argument("--debug",  help="1: print weights and field 2: make overlap check", required=False, default=0, type=int, choices=range(0,3))
-parser.add_argument("--helium",  help="use helium bag for vacuum tank", required=False, action="store_const", const="helium", default="vacuums")
+parser.add_argument(
+     "--helium",
+     help="Replace vacuum with helium (vessel structure unchanged)",
+     action="store_const",
+     const="helium",
+     default="vacuums"
+)
 
 options = parser.parse_args()