Remove a mutable requirement.

How was this not an issue before?

benches/bench.rs

@@ -77,7 +77,7 @@ fn fee(c: &mut Criterion) {
         let delays = [0; 16];
         let gains = [1.0; 16];
         let norm_to_zenith = false;
-        let mut beam = FEEBeam::new("mwa_full_embedded_element_pattern.h5").unwrap();
+        let beam = FEEBeam::new("mwa_full_embedded_element_pattern.h5").unwrap();
         // Prime the cache.
         beam.calc_jones(az[0], za[0], freq, &delays, &gains, norm_to_zenith)
             .unwrap();

examples/beam_calcs.rs

@@ -35,7 +35,7 @@ struct Opts {
 fn main() -> Result<(), anyhow::Error> {
     let opts = Opts::from_args();
     // If we were given a file, use it. Otherwise, fall back on MWA_BEAM_FILE.
-    let mut beam = match opts.hdf5_file {
+    let beam = match opts.hdf5_file {
         Some(f) => FEEBeam::new(f)?,
         None => FEEBeam::new_from_env()?,
     };

src/fee/mod.rs

@@ -409,7 +409,7 @@ impl FEEBeam {
     /// calculates the Jones matrices in parallel. `delays` and `amps` *must*
     /// have 16 elements.
     pub fn calc_jones_array(
-        &mut self,
+        &self,
         az_rad: &[f64],
         za_rad: &[f64],
         freq_hz: u32,