Derived quantity objects data and t attributes are not emptied at beginning of simulation

[RemDelaporteMathurin]

Describe the bug
When running the same simulation twice with derived quantities, their t and data attributes isn't reset to an empty list but instead values are just appended.

To Reproduce
Run the following on fenicsx

import festim as F

my_model = F.HydrogenTransportProblem()

my_model.mesh = F.Mesh1D([0, 1, 2, 3, 4])

tungsten = F.Material(D_0=1, E_D=0)

my_subdomain = F.VolumeSubdomain1D(id=1, borders=[0, 4], material=tungsten)
left_surface = F.SurfaceSubdomain1D(id=1, x=0)

my_model.subdomains = [my_subdomain, left_surface]

mobile_H = F.Species("H")
my_model.species = [mobile_H]

my_model.temperature = 400

left_flux = F.SurfaceFlux(field=mobile_H, surface=left_surface)

my_model.exports = [left_flux]

my_model.settings = F.Settings(
    atol=1e-15,
    rtol=1e-15,
    transient=False,
)

for i in range(3):
    my_model.initialise()
    my_model.run()

    print(left_flux.data)

Produces:

[0.0]
[0.0, 0.0]
[0.0, 0.0, 0.0]

Expected behavior
left_flux.data should be of length 1 everytime

[RemDelaporteMathurin]

I noticed this bug also occurs on the main branch!