cross: add Real::c_inv()

crates/cross/src/real.rs

@@ -138,6 +138,7 @@ pub trait Real:
     /// Speed of light in a vacuum; m/s.
     /// Also equal to 1/sqrt(epsilon_0 mu_0)
     fn c() -> Self;
+    fn c_inv() -> Self;
     /// Vaccum Permittivity
     fn eps0() -> Self;
     fn eps0_inv() -> Self;
@@ -189,6 +190,9 @@ macro_rules! decl_consts {
         fn c() -> Self {
             $wrap(299792458.0)
         }
+        fn c_inv() -> Self {
+            $wrap(3.3356409519815204e-09)
+        }
         /// Vaccum Permittivity
         fn eps0() -> Self {
             $wrap(8.854187812813e-12) // F/m
@@ -501,6 +505,9 @@ impl<T: Real> Real for Finite<T> {
     fn c() -> Self {
         Self::from_primitive(T::c())
     }
+    fn c_inv() -> Self {
+        Self::from_primitive(T::c_inv())
+    }
     fn eps0() -> Self {
         Self::from_primitive(T::eps0())
     }