provide Stepsize as float or int

[jhdark]

Proposed changes

Fix for #846

With this change users can define the dt simply by an int or float such as:

import festim as F

my_model = F.Simulation()

my_model.mesh = F.MeshFromVertices([1, 2, 3, 4])

my_model.materials = F.Material(id=1, D_0=1, E_D=0)
my_model.T = 300
my_model.settings = F.Settings(
    absolute_tolerance=1e-10,
    relative_tolerance=1e-10,
    transient=True,
    final_time=10,
)

my_model.dt = 1    # <------- provide stepsize as int or float


my_model.initialise()
my_model.run()

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)

Further comments

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did and what alternatives you considered, etc...

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.26%. Comparing base (8d6857b) to head (59c2eb4).
Report is 67 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #936      +/-   ##
==========================================
- Coverage   99.27%   99.26%   -0.02%     
==========================================
  Files          60       60              
  Lines        2753     2713      -40     
==========================================
- Hits         2733     2693      -40     
  Misses         20       20              

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[RemDelaporteMathurin]

@jhdark can you format the code and then feel free to merge