Add scripts for creating a system image to speed up GUI loading

Running PersefoneDesktop with this custom system image, latencies are
reduced as follows:

- startup time of the GUI (until the map is shown): 50s (previously 95s)

- showing the population graph window: <1s (previously multiple seconds)

The system image is created with `sysimg_create.jl`.

To use the system image, run `julia -J sysimg.so desktop.jl`.

The majority of the remaining startup time (45s of 50s) is due to the
initialisation of the Persefone simulation.  This might be reduced by
further improving persefone-model.

.gitignore

@@ -6,3 +6,5 @@ examples
 # environment.
 Manifest.toml
 
+# system image used to speed up GUI loading
+/sysimg.so
\ No newline at end of file

sysimg_create.jl

+#!/usr/bin/env julia
+
+import Pkg
+Pkg.activate(".")
+
+using PackageCompiler
+
+# Creating the sysimg takes a few minutes and uses around 13GB of RAM.
+#
+# TODO: currently all GUI functions must be called manually, optimally
+# every GUI function is called and a complete simulation is run.
+create_sysimage(["Persefone", "CSV", "CxxWrap", "DataFramesMeta", "FileIO", "GLMakie",
+                 "GeometryBasics", "Observables", "QML", "ResumableFunctions", "TableTransforms", "TOML"],
+                sysimage_path="sysimg.so",
+                precompile_execution_file="sysimg_precompile.jl")

sysimg_precompile.jl

+using PersefoneDesktop
+# TODO: currently all important GUI functions that should be
+# precompiled must be called manually, and then the GUI must be closed
+# manually. It would be good if this could be completely automated.
+launch()
+