Make regulator naming more consistent

Although the inversive distance regulator mostly acts as a general
Lorentz product regulator, calling it `InversiveDistanceRegulator`
explains how we intend to use it and demonstrates our naming convention.

app-proto/src/assembly.rs

@@ -151,14 +151,14 @@ pub trait Regulator: ProblemPoser + OutlineItem {
     fn activate(&self, _assembly: &Assembly) {}
 }
 
-pub struct ProductRegulator {
+pub struct InversiveDistanceRegulator {
     pub subjects: [ElementKey; 2],
     pub measurement: ReadSignal<f64>,
     pub set_point: Signal<SpecifiedValue>
 }
 
-impl ProductRegulator {
-    pub fn new(subjects: [ElementKey; 2], assembly: &Assembly) -> ProductRegulator {
+impl InversiveDistanceRegulator {
+    pub fn new(subjects: [ElementKey; 2], assembly: &Assembly) -> InversiveDistanceRegulator {
         let measurement = assembly.elements.map(
             move |elts| {
                 let representations = subjects.map(|subj| elts[subj].representation);
@@ -172,7 +172,7 @@ impl ProductRegulator {
         
         let set_point = create_signal(SpecifiedValue::from_empty_spec());
         
-        ProductRegulator {
+        InversiveDistanceRegulator {
             subjects: subjects,
             measurement: measurement,
             set_point: set_point
@@ -180,7 +180,7 @@ impl ProductRegulator {
     }
 }
 
-impl Regulator for ProductRegulator {
+impl Regulator for InversiveDistanceRegulator {
     fn subjects(&self) -> Vec<ElementKey> {
         self.subjects.into()
     }
@@ -194,7 +194,7 @@ impl Regulator for ProductRegulator {
     }
 }
 
-impl ProblemPoser for ProductRegulator {
+impl ProblemPoser for InversiveDistanceRegulator {
     fn pose(&self, problem: &mut ConstraintProblem, elts: &Slab<Element>) {
         self.set_point.with_untracked(|set_pt| {
             if let Some(val) = set_pt.value {
@@ -633,7 +633,7 @@ mod tests {
                 )
             });
             elts[subjects[0]].column_index = Some(0);
-            ProductRegulator {
+            InversiveDistanceRegulator {
                 subjects: subjects,
                 measurement: create_memo(|| 0.0),
                 set_point: create_signal(SpecifiedValue::try_from("0.0".to_string()).unwrap())