meas: backfill tests for CurrentLoop

crates/cross/src/step.rs

@@ -20,6 +20,17 @@ pub struct SimMeta<R> {
     pub feature_size: R,
 }
 
+impl<R: Real> SimMeta<R> {
+    pub fn new(dim: Vec3u, feature_size: R, time_step: R) -> Self {
+        Self {
+            dim,
+            inv_feature_size: feature_size.inv(),
+            time_step,
+            feature_size
+        }
+    }
+}
+
 impl<R: Copy> SimMeta<R> {
     pub fn dim(&self) -> Vec3u {
         self.dim