General(solver).units = UnitSystem General(solver).units[DIMENSION] = Unit

### 📝 Description of the feature

I'm not quite set on whether this should be on the units object directly or if you should have to do `General(solver).units.units = C` or similar for unit system, to me it feels overly verbose.

In general though the current state of this is very verbose
```py
General(solver).units.set_units(
quantity="temperature", units_name="C", scale_factor=1.0, offset=273.15
)
```
Currently trying to do this will raise
```py
>>> General(solver).units.units = C

Error: CAR: invalid argument [1]: wrong type [not a pair]
Error Object: -249804576

Error: CAR: invalid argument [1]: wrong type [not a pair]
Error Object: -249804576
Traceback (most recent call last):
File "", line 1, in
File "C:\Program Files\ANSYS Inc\v261\commonfiles\CPython\3_10\winx64\Release\Ansys\PyFluentCore\ansys\fluent\core\solver\flobject.py", line 1229, in __setattr__
return attr.set_state(value)
File "C:\Program Files\ANSYS Inc\v261\commonfiles\CPython\3_10\winx64\Release\Ansys\PyFluentCore\ansys\fluent\core\solver\flobject.py", line 857, in set_state
self.flproxy.set_var(
File "C:\PROGRA~1\ANSYSI~1\v261\fluent\fluent26.1.0\cortex\pylib\flapi\flproxy.py", line 16, in set_var
return scheme.fl_set_var(path, value)
File "C:\PROGRA~1\ANSYSI~1\v261\fluent\fluent26.1.0\cortex\pylib\flapi\scheme.py", line 27, in newf
return f(*args, **kwds)
File "C:\PROGRA~1\ANSYSI~1\v261\fluent\fluent26.1.0\cortex\pylib\flapi\scheme.py", line 115, in fl_set_var
return _fl_set_var(path, value)
RuntimeError: CAR: invalid argument [1]: wrong type [not a pair]
Error Object: -249804576
```

### 💡 Steps for implementing the feature

Unsure

### 🔗 Useful links and references

_No response_

Start with the General(solver).units object and its current set_units(quantity, units_name, scale_factor, offset) interface, then reproduce the reported General(solver).units.units = C failure. Done should be a decided, documented unit-system assignment API that accepts the intended value without the CAR type error.

Written by the indexing model from the issue text.