Parallelize generate testmodels (#1309)

Fixes #1302

On my machine, that reduces the runtime of `pixi run
generate-testmodels` from 1m50s to 42s

utils/generate-testmodels.py

@@ -1,8 +1,18 @@
+import multiprocessing
 import shutil
+from functools import partial
 from pathlib import Path
 
 import ribasim_testmodels
 
+
+def generate_model(args, datadir):
+    model_name, model_constructor = args
+    model = model_constructor()
+    model.write(datadir / model_name / "ribasim.toml")
+    return model_name
+
+
 if __name__ == "__main__":
     datadir = Path("generated_testmodels")
     if datadir.is_dir():
@@ -18,7 +28,10 @@
 Don't put important stuff in here, it will be emptied for every run."""
     )
 
-    for model_name, model_constructor in ribasim_testmodels.constructors.items():
-        print(f"Generating {model_name}")
-        model = model_constructor()
-        model.write(datadir / model_name / "ribasim.toml")
+    generate_model_partial = partial(generate_model, datadir=datadir)
+
+    with multiprocessing.Pool(processes=4) as p:
+        for model_name in p.imap_unordered(
+            generate_model_partial, ribasim_testmodels.constructors.items()
+        ):
+            print(f"Generated {model_name}")