Bug with atol in KLU

[valentinsulzer]

Describe the bug
Cannot do convergence study with IDAKLUSolver due to error with atol

To Reproduce
Run the following code:

import pybamm
import numpy as np


model = pybamm.lithium_ion.SPM()
solver = pybamm.IDAKLUSolver()

# load parameter values and geometry
geometry = model.default_geometry
param = model.default_parameter_values

# Process parameters
param.process_model(model)
param.process_geometry(geometry)

# Calculate time for each solver and each number of grid points
var = pybamm.standard_spatial_vars
t_eval = np.linspace(0, 0.17, 100)
for npts in [100, 200]:
    # discretise
    var_pts = {
        spatial_var: npts
        for spatial_var in [var.x_n, var.x_s, var.x_p, var.r_n, var.r_p]
    }
    mesh = pybamm.Mesh(geometry, model.default_submesh_types, var_pts)
    disc = pybamm.Discretisation(mesh, model.default_spatial_methods)
    model_disc = disc.process_model(model, inplace=False)

    # solve
    solution = solver.solve(model_disc, t_eval)

Expected behaviour
No output

Actual behaviour
It goes through the first solve fine, but fails on the second solve

Traceback (most recent call last):
  File "/mnt/c/Users/vsulzer/Documents/Energy_Storage/PyBaMM/bug.py", line 30, in <module>
    solution = solver.solve(model_disc, t_eval)
  File "/mnt/c/Users/vsulzer/Documents/Energy_Storage/PyBaMM/pybamm/solvers/base_solver.py", line 84, in solve
    solution, solve_time, termination = self.compute_solution(model, t_eval)
  File "/mnt/c/Users/vsulzer/Documents/Energy_Storage/PyBaMM/pybamm/solvers/dae_solver.py", line 90, in compute_solution
    jacobian=self.jacobian,
  File "/mnt/c/Users/vsulzer/Documents/Energy_Storage/PyBaMM/pybamm/solvers/idaklu_solver.py", line 158, in integrate
    self._check_atol_type(y0.size)
  File "/mnt/c/Users/vsulzer/Documents/Energy_Storage/PyBaMM/pybamm/solvers/idaklu_solver.py", line 124, in _check_atol_type
    of the same shape at y0"""
pybamm.expression_tree.exceptions.SolverError: Absolute tolerances must be either a scalar or a numpy arrray
                of the same shape at y0

[valentinsulzer]

Note this is fixed by making _check_atol_type return atol directly (instead of setting self._atol and then reading atol=self._atol, but you might want to do it differently?