stimulus: add a structure to sum together two stimuli of different types

this may resemble the original List stuff. only it's more specialized,
for only summing two of a thing.

crates/coremem/src/stim.rs

@@ -482,6 +482,9 @@ pub trait StimExt<R>: Sized {
     fn scaled<T: TimeVarying<R>>(self, scale: T) -> Scaled<Self, T> {
         Scaled::new(self, scale)
     }
+    fn summed<T: TimeVarying<R>>(self, with: T) -> Summed<Self, T> {
+        Summed::new(self, with)
+    }
 }
 
 impl<R, T> StimExt<R> for T {}
@@ -665,6 +668,21 @@ impl<R: Real, A: TimeVarying<R>, B: TimeVarying<R>> TimeVarying<R> for Scaled<A,
     }
 }
 
+#[derive(Clone)]
+pub struct Summed<A, B>(A, B);
+
+impl<A, B> Summed<A, B> {
+    pub fn new(a: A, b: B) -> Self {
+        Self(a, b)
+    }
+}
+
+impl<R: Real, A: TimeVarying<R>, B: TimeVarying<R>> TimeVarying<R> for Summed<A, B> {
+    fn at(&self, t_sec: R) -> FieldMags<R> {
+        self.0.at(t_sec) + self.1.at(t_sec)
+    }
+}
+
 #[cfg(test)]
 mod test {
     use super::*;