fix: Correct scale factor on last half step

There is a missing `; a = a(t)` in the argument to `outer_step!` that
does the closing half step.  This is never an issue if there is no
background expansion.

Add it in.

This should maybe not be an optional argument; it shouldn't be possible
to accidentally call with the wrong `a(t)`.

Author: musoke

Project.toml

@@ -1,7 +1,7 @@
 name = "UltraDark"
 uuid = "1c8d022d-dfc0-4b41-80ab-3fc7e88cdfea"
 authors = ["Nathan Musoke <n.musoke@auckland.ac.nz>"]
-version = "0.9.2"
+version = "0.9.3"
 
 [deps]
 AbstractFFTs = "621f4979-c628-5d54-868e-fcf4e3e8185c"

src/UltraDark.jl

@@ -165,7 +165,7 @@ function take_steps!(grids, t_start, Δt, n, output_config, a, constants, extern
         output_summary_row(grids, output_config, t, a(t), Δt, constants, external_states)
     end
 
-    outer_step!(Δt / 2, grids, constants)
+    outer_step!(Δt / 2, grids, constants; a = a(t))
     for s in external_states
         outer_step!(Δt / 2, grids, constants, s; a = a(t))
     end