Add OPPOSSUM_FILE_VERSION to build system

.cargo/config.toml

+[env]
+OPM_FILE_VERSION = "0"

Cargo.toml

@@ -9,7 +9,6 @@ license = "GPL-3.0-or-later"
 keywords = ["laser", "optics", "physics", "simulation"]
 categories = [ "science", "simulation"]
 publish = false
-
 build = "build.rs"
 
 [dependencies]

src/optic_scenery.rs

@@ -257,6 +257,9 @@ impl OpticScenery {
         serde_json::Value::Object(report)
     }
     pub fn save_to_file(&self, path: &Path) -> OpmResult<()> {
+         // testing only:
+        let file_version=env!("OPM_FILE_VERSION");
+        println!("file version: {:?}", file_version);
         let serialized = serde_json::to_string_pretty(&self).map_err(|e| {
             OpossumError::OpticScenery(format!("deserialization of OpticScenery failed: {}", e))
         })?;