Add error message when no final time is provided in transient simulations

[RemDelaporteMathurin]

Proposed changes

As described in this Discourse topic when users don't provide final_time to settings when running a transient simulation, the error is not explanatory.

This PR simply adds a check to initialise() and raises a dedicated error message.

Types of changes

What types of changes does your code introduce to FESTIM?

• [ ] Bugfix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Code refactoring
• [ ] Documentation Update (if none of the other choices apply)
• [ ] New tests

Checklist

• [x] Black formatted
• [x] Unit tests pass locally with my changes
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] I have added necessary documentation (if appropriate)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.51%. Comparing base (315aebd) to head (38677a7).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #769 +/- ## ======================================= Coverage 99.51% 99.51% ======================================= Files 58 58 Lines 2476 2478 +2 ======================================= + Hits 2464 2466 +2 Misses 12 12 ```

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