Grid Editor - Problem Saving Core Map

@jakehader Reported this bug: given the right conditions (which are common), the grid editor is failing to write output files.

Steps to Reproduce the Bug

1. From the current “master” branch, open the grid editor on the command line: $ armi grids
2. Load this blueprints YAML file: test-full-bp.zip
3. Hit the buttons “Expand to full core” >> “Apply” >> “Save to grid blueprints…”
4. This will yield a valid output YAML file that looks like:

core:
  geom: hex
  lattice map:
  grid bounds:
  symmetry: full
  grid contents:
    ? - 0
      - 0
    : IF

1. Now, add full ring of 6 "inner fuel"s around the central hex. and again “Save to grid blueprints…” You will now see the broken output YAML file:

core:
  geom: hex
  lattice map: |-
    None
  grid bounds:
  symmetry: full
  grid contents:

The Bug

The second core map above is wrong because it has no output data.

What Caused this Bug?

Jake and determined this bug entered the “master” branch at this PR: https://github.com/terrapower/armi/pull/439

[ntouran]

I’m working to make a unit test that would have caught this issue.

Ok so we still do get an exception when we run the reproduction example but with the exception no longer being hidden:

The `lattice map` for the current assembly arrangement cannot be written. Defaulting to using the `grid contents` dictionary instead. Exception: max() arg is an empty sequence

This is very likely coming from asciimaps._updateDimensionsFromData. That may help us find the root cause.

[jakehader]

Fix in #494