QoL: provide stepsize as float

[RemDelaporteMathurin]

Users should be able to do:

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()

And not provide the stepsize as a F.Stepsize object for simple cases like this.

To do so, all we need is to write a setter function in Simulation

    @property
    def dt(self):
        return self._dt

    @dt.setter
    def dt(self, value):
        if isinstance(value, (int, float)):
            self._dt = F.Stepsize(value)
        elif isinstance(value, F.Stepsize):
            self._dt = value
        else:
            raise TypeError("dt must be an int, float, or festim.Stepsize")