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)
• 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

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.51%. Comparing base (315aebd) to head (38677a7).
Report is 225 commits behind head on main.

Additional details and impacted files

@@           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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[jhdark]

Looks good to me