Add hepmcmerger (#647)

### Briefly, what does this PR introduce?
Add a package for https://github.com/eic/HEPMC_Merger

### What kind of change does this PR introduce?
- [ ] Bug fix (issue #__)
- [x] New feature (issue #__)
- [ ] Documentation update
- [ ] Other: __

### Please check if this PR fulfills the following:
- [ ] Tests for the changes have been added
- [ ] Documentation has been added / updated
- [ ] Changes have been communicated to collaborators

### Does this PR introduce breaking changes? What changes might users
need to make to their code?
No

### Does this PR change default behavior?
No

---------

Co-authored-by: Wouter Deconinck <[email protected]>

packages/hepmcmerger/package.py

@@ -0,0 +1,36 @@
+# Copyright 2013-2024 Lawrence Livermore National Security, LLC and other
+# Spack Project Developers. See the top-level COPYRIGHT file for details.
+#
+# SPDX-License-Identifier: (Apache-2.0 OR MIT)
+
+
+from spack.package import *
+
+
+class Hepmcmerger(CMakePackage):
+    """An EIC HepMC merger to combine signal and background events."""
+
+    homepage = "https://github.com/eic/HEPMC_Merger"
+    url = "https://github.com/eic/HEPMC_Merger/archive/refs/tags/v1.0.4.tar.gz"
+    list_url = "https://github.com/eic/HEPMC_Merger/tags"
+    git = "https://github.com/eic/HEPMC_Merger.git"
+
+    maintainers("kkauder")
+
+    tags = ["eic"]
+
+    version("main", branch="main")
+    version("1.0.4", sha256="0f5e1d6b2d76af764f5cc528ccfad1269047f6e361b6bc0a1b80941388f71437")
+    version("1.0.3", sha256="9f245dc46e159f9424383a4337ebbc685973e83240fe178c776e6cca5e9674f0")
+    version("1.0.2", sha256="e8bb639545e472f46b7de8f0c6e03c9ee61086c92b8ffcba661f0fe3b1064ad6")
+    version("1.0.1", sha256="419732c2d46afbad89e32362d339a643dc1e6e5ff9724c3027a45aef1b8fbf95")
+    version("1.0.0", sha256="5f36b0b65f1062aab79dc6653b6f6fecb9682022f1a471efa62b5614c9731618")
+
+    depends_on("hepmc3")
+    depends_on("root")
+
+    def cmake_args(self):
+        args = [
+            self.define("CMAKE_CXX_STANDARD", self.spec["root"].variants["cxxstd"].value),
+        ]
+        return args