[BUG] Cannot export to VTX with multispecies

Describe the bug When exporting to VTX with multispecies, it just exports zero everywhere. The correct output is given for XDMF

To Reproduce Run the following on the fenicsx branch

import festim as F
import numpy as np

L = 3e-04
vertices = np.linspace(0, L, num=1000)

my_model = F.HydrogenTransportProblem()
my_model.mesh = F.Mesh1D(vertices)

my_mat = F.Material(D_0=1.9e-7, E_D=0.2, name="my_mat")
my_subdomain = F.VolumeSubdomain1D(id=1, borders=[0, L], material=my_mat)
left_surface = F.SurfaceSubdomain1D(id=1, x=0)
right_surface = F.SurfaceSubdomain1D(id=2, x=L)
my_model.subdomains = [
    my_subdomain,
    left_surface,
    right_surface,
]

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

temperature = 500.0
my_model.temperature = temperature

my_model.boundary_conditions = [
    F.DirichletBC(subdomain=right_surface, value=0, species=mobile_H),
    F.DirichletBC(subdomain=left_surface, value=1e12, species=mobile_H),
]
my_model.exports = [
    F.VTXExport("mobile_concentration_h.bp", field=mobile_H),
    F.VTXExport("trapped_concentration_h.bp", field=trapped_H),
    F.XDMFExport("mobile_concentration_h.xdmf", field=mobile_H),
    F.XDMFExport("trapped_concentration_h.xdmf", field=trapped_H),
]

my_model.settings = F.Settings(
    atol=1e10,
    rtol=1e-10,
    max_iterations=30,
    final_time=50,
)

my_model.settings.stepsize = F.Stepsize(initial_value=1 / 20)

my_model.initialise()

from petsc4py import PETSc

my_model.solver.convergence_criterion = "incremental"
ksp = my_model.solver.krylov_solver
opts = PETSc.Options()
option_prefix = ksp.getOptionsPrefix()
opts[f"{option_prefix}ksp_type"] = "cg"
opts[f"{option_prefix}pc_type"] = "gamg"
opts[f"{option_prefix}pc_factor_mat_solver_type"] = "mumps"
ksp.setFromOptions()

my_model.run()

festim-dev / FESTIM

[BUG] Cannot export to VTX with multispecies #622