app: multi_core_inverter: perf: move the stimulus Gating to outside the CurlStimulus

the region.contains() logic is much more expensive than the time bounds
check.
this gets an easy 50% perf boost to the ENTIRE simulation

crates/coremem/src/stim.rs

@@ -346,6 +346,17 @@ impl TimeVarying3 for Exp3 {
     }
 }
 
+impl<A: AbstractStimulus> AbstractStimulus for Exp<A> {
+    fn at(&self, t_sec: f32, pos: Meters) -> Fields {
+        let scale = (t_sec * -self.tau).exp();
+        let inner = self.amp.at(t_sec, pos);
+        Fields {
+            e: inner.e * scale,
+            h: inner.h * scale,
+        }
+    }
+}
+
 #[derive(Clone)]
 pub struct Gated<T> {
     inner: T,
@@ -380,6 +391,16 @@ impl<T: TimeVarying3> TimeVarying3 for Gated<T> {
     }
 }
 
+impl<T: AbstractStimulus> AbstractStimulus for Gated<T> {
+    fn at(&self, t_sec: f32, pos: Meters) -> Fields {
+        if (self.start..self.end).contains(&t_sec) {
+            self.inner.at(t_sec, pos)
+        } else {
+            Default::default()
+        }
+    }
+}
+
 #[derive(Clone)]
 pub struct Shifted<T> {
     inner: T,
@@ -405,6 +426,12 @@ impl<T: TimeVarying3> TimeVarying3 for Shifted<T> {
     }
 }
 
+impl<T: AbstractStimulus> AbstractStimulus for Shifted<T> {
+    fn at(&self, t_sec: f32, pos: Meters) -> Fields {
+        self.inner.at(t_sec - self.start_at, pos)
+    }
+}
+
 #[cfg(test)]
 mod test {
     use super::*;