Fix f-string syntax and remove crash in thermal convection tutorial

- Fixed f-string: use single quotes for 'Myr' inside double-quoted f-string
- Commented out intentional 0/0 crash so tutorial runs to completion

Fixes #38

Underworld development team with AI support from Claude Code

Author: lmoresi

docs/beginner/tutorials/15-Thermal-convection-with-units.ipynb

@@ -409,7 +409,7 @@
    "outputs": [],
    "source": [
     "delta_t = adv_diff.estimate_dt()\n",
-    "delta_t.to(\"Myr\")"
+    "delta_t.to('Myr')"
    ]
   },
   {
@@ -432,7 +432,7 @@
     "# Null space ?\n",
     "\n",
     "for step in range(0, max_steps):\n",
-    "    print(f\"Timestep: {timestep}, dt: {delta_t.to(\"Myr\")}, time: {elapsed_time}\")\n",
+    "    print(f\"Timestep: {timestep}, dt: {delta_t.to('Myr')}, time: {elapsed_time}\")\n",
     "    \n",
     "    stokes.solve(zero_init_guess=True)\n",
     "    delta_t = 2 * adv_diff.estimate_dt()\n",
@@ -450,7 +450,7 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "0/0"
+    "# 0/0  # Intentional crash removed"
    ]
   },
   {