Remove outdated v3 code

[VeckoTheGecko]

Fixes #2255
Changes:

• Rename interpolation_utils.py to place in _core/utils/interpolation.py
• Remove timer.py
• Remove interpolators.py
• Move some tests, remove unused utils

Tree difference for src/parcels:

--- old-tree.txt	2025-11-03 14:00:28
+++ new-tree.txt	2025-11-03 14:00:37
@@ -16,6 +16,7 @@
 │   ├── statuscodes.py
 │   ├── utils
 │   │   ├── __init__.py
+│   │   ├── interpolation.py
 │   │   ├── string.py
 │   │   ├── time.py
 │   │   └── unstructured.py
@@ -34,7 +35,6 @@
 │   │   └── generic.py
 │   └── utils.py
 ├── _decorators.py
-├── _interpolation.py
 ├── _logger.py
 ├── _python.py
 ├── _reprs.py
@@ -63,11 +63,6 @@
 │   ├── advection.py
 │   ├── advectiondiffusion.py
 │   └── interaction.py
-├── tools
-└── utils
-    ├── __init__.py
-    ├── _helpers.py
-    ├── interpolation_utils.py
-    └── timer.py
+└── tools
 
-13 directories, 58 files
+12 directories, 54 files

[VeckoTheGecko]

This is probably easiest to review commit by commit

[erikvansebille]

Looks good!