ion_map_ids: static & const access (#2010)

This fixes the `ion_map_ids` atom-to-number object:
- declare static, so the file can be included in multiple
  translation units
- use `.at()`:
  - missing entries do not cause an insertion
  - missing entries will throw
  - access is `const`, so we can declare the whole object
    `static const` as well.

1 files changed, 1 insertions, 1 deletions

diff --git a/Source/Particles/PhysicalParticleContainer.cpp b/Source/Particles/PhysicalParticleContainer.cpp
index a8140d4f4..ae7111550 100644
--- a/Source/Particles/PhysicalParticleContainer.cpp
+++ b/Source/Particles/PhysicalParticleContainer.cpp
@@ -2324,7 +2324,7 @@ PhysicalParticleContainer::InitIonizationModule ()
     // Add runtime integer component for ionization level
     AddIntComp("ionization_level");
     // Get atomic number and ionization energies from file
-    int ion_element_id = ion_map_ids[physical_element];
+    int const ion_element_id = ion_map_ids.at(physical_element);
     ion_atomic_number = ion_atomic_numbers[ion_element_id];
     Vector<Real> h_ionization_energies(ion_atomic_number);
     int offset = ion_energy_offsets[ion_element_id];