Fix thermal convection units tutorial

[ss2098]

This updates docs/examples/Tutorial_Thermal_Convection_Units.py so that it runs with the current unit-aware array and strict-units behavior.

The previous tutorial failed in several places under the current API:

1. The buoyancy expression mixed a MeshVariable symbolic field with a unit-bearing reference temperature, producing a dimensional compatibility error.
2. The initial temperature assignment attempted to assign unit-aware evaluate() output directly into MeshVariable storage, which required explicit conversion to a plain numeric array.
3. The semi-Lagrangian advection step encountered incompatible coordinate and timestep units unless the velocity field carried physical velocity units.
4. The final reporting attempted to convert model-unit temperature quantities directly to offset Celsius units, which raised a Pint OffsetUnitCalculusError.
5. The initial temperature profile could become incorrectly scaled because unit-aware coordinates needed to be normalized before constructing the model-unit temperature field.

The updated tutorial keeps the physical inputs in units, but uses explicit model-unit numeric values in solver expressions. It also converts unit-aware arrays before assignment, constructs the initial temperature field from normalized coordinates, and reports original physical parameters for interpretation.

No solver behavior is changed.

Tested with:

pixi run python docs/examples/Tutorial_Thermal_Convection_Units.py

Result: script completes successfully.