Only compile engine::point when it's used

This function will eventually be used in the application, but right now
it's only used in tests.

app-proto/src/engine.rs

@@ -4,6 +4,7 @@ use web_sys::{console, wasm_bindgen::JsValue}; /* DEBUG */
 
 // --- elements ---
 
+#[cfg(test)]
 pub fn point(x: f64, y: f64, z: f64) -> DVector<f64> {
     DVector::from_column_slice(&[x, y, z, 0.5, 0.5*(x*x + y*y + z*z)])
 }