NpT sanity checks should run at integration start

[jngrad]

The Coulomb/dipoles NpT sanity checks are executed only once, when setting up the NpT integrator. Adding a P3M actor before setting a rectangular NpT integrator fails loudly. But if we add a P3M actor after setting up a rectangular NpT, no error is raised.

MWE:

import espressomd.integrate
import espressomd.electrostatics
import numpy as np
np.random.seed(42)

system = espressomd.System(box_l=3 * [6], time_step = 0.01)
system.cell_system.skin = 0.25
system.part.add(pos=np.random.uniform(0, system.box_l[0], (10, 3)), q=5 * [-1, 1])
system.non_bonded_inter[0, 0].wca.set_params(epsilon=1, sigma=1)
npt_params = dict(ext_pressure=1e-4, piston=1e-4, cubic_box=False, direction=(0, 1, 1))
system.thermostat.set_npt(kT=1.0, gamma0=2, gammav=0.04, seed=42)
system.integrator.set_isotropic_npt(**npt_params)
system.actors.add(espressomd.electrostatics.P3M(prefactor=1.0, accuracy=1e-2,
                  mesh=3 * [8], cao=3, r_cut=0.36, alpha=5.35, tune=False))
system.integrator.run(100)     # no failure
print('box_l:', system.box_l)  # box is now rectangular
system.integrator.set_isotropic_npt(**npt_params)  # fails

Output:

box_l: [ 6. 18.34 18.34]
Traceback (most recent call last):
  File "npt.py", line 18, in <module>
    system.integrator.set_isotropic_npt(**npt_params)  # fails
  File "integrate.pyx", line 92, in espressomd.integrate.IntegratorHandle.set_isotropic_npt
  File "integrate.pyx", line 147, in espressomd.integrate.Integrator.__init__
  File "integrate.pyx", line 364, in espressomd.integrate.VelocityVerletIsotropicNPT._set_params_in_es_core
RuntimeError: If electrostatics is being used you must use the cubic box npt.

This can be fixed by moving the sanity checks from the setter to the callback.