Issue with assigning ICs in separate volumes for Heat Transfer Problem

**Describe the bug**
FESTIM version 2.1
In FESTIM docs tutorials section "Initial conditions -> Temperature", initial temperatures are set using:
```python
model.initial_conditions = [
F.InitialTemperature(value=300, volume=vol)
]
```
however when attempting to assign initial conditions to a heat transfer model, only the attribute "initial_condition" is available, not the plural, which only allows a single initial condition to be set within a single volume, whereas documentation suggests initial conditions can be set in a list for discontinuous problems with multiple volume subdomains. Setting model.initial_conditions does not throw an error despite not actually setting any conditions.

**To Reproduce**
Steps to reproduce the behaviour:

Set e.g.
```python
model.initial_conditions = [
F.InitialTemperature(value=temp, volume=vol1),
F.InitialTemperature(value=temp, volume=vol2),
F.InitialTemperature(value=temp, volume=vol3),
]
```
and print
```
model.u_n.x.array.min(),
model.u_n.x.array.max()
```
after model.initialise() and before model.run() which will return "0.0 0.0", whereas when using initial_condition returns "temp temp" (i.e. the value of temp, e.g. 300.0 if temp=300.0)

Also attempt to set multiple volumes to a single initial condition
```
model.initial_condition = F.InitialTemperature(value=temp, volume=model.volume_subdomains)
```

which will raise an error
```
TypeError("volume must be of type festim.VolumeSubdomain")
```
**Expected behavior**
Expected to be able to set initial conditions in a list for multiple volume subdomains using model.initial_conditions = [], as described within "https://festim-workshop.readthedocs.io/en/latest/content/initial_conditions/temperature.html"

Start with the initial-conditions temperature tutorial linked in the issue and reproduce the model.u_n.x.array check using multiple volume subdomains. Compare the documented initial_conditions list with the model's available initial_condition handling and volume validation. Done means multiple volume-specific temperatures are applied after initialise() and the documentation matches the supported behavior.

Written by the indexing model from the issue text.