CurlStimulus: take axis and center directly from the Region

by taking more from the region, we'll be able to reuse common code
and also make this more testable

crates/applications/buffer_proto5/src/main.rs

@@ -429,8 +429,6 @@ fn run_sim(id: u32, p: Params, g: Geometries) -> Results {
         driver.add_stimulus(CurlStimulus::new(
             region.clone(),
             wave.clone(),
-            region.center(),
-            region.axis()
         ));
     };
 
@@ -439,8 +437,6 @@ fn run_sim(id: u32, p: Params, g: Geometries) -> Results {
         driver.add_stimulus(CurlStimulus::new(
             region.clone(),
             wave.clone(),
-            region.center(),
-            region.axis()
         ));
     };
 
@@ -449,8 +445,6 @@ fn run_sim(id: u32, p: Params, g: Geometries) -> Results {
         driver.add_stimulus(CurlStimulus::new(
             region.clone(),
             wave.clone(),
-            region.center(),
-            region.axis()
         ));
     };