`GetParticles(lev)` causes 'Segfault' when there is no particle at level `lev`

[chongchonghe]

To reproduce:

TestParticleContainer = amrex::AmrParticleContainer<3, 1>;

// some operations to add particles at level 1 but keep level 0 empty

auto &particles = TestParticleContainer->GetParticles(0);

See Segfault.

[BenWibking]

@atmyers Let us know if you can provide a fix.

[atmyers]

If you add a call to the resizeData() method of ParticleContainer right after before you create the initial particles, does this fix your problem?