Fix the gradient energy calculation

[jijn]

• Please check if the PR fulfills these requirements

• The PR follows our guidelines (formatting & style)
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
  Fix the gradient energy calculation for a scalar gradient coefficient. This fixes Gradient energy calculation seems wrong #769.

• What is the current behavior? (You can also link to an open issue here)
  See Gradient energy calculation seems wrong #769

• What is the new behavior (if this is a feature change)?
  See Gradient energy calculation seems wrong #769

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)
  No

• Other information:
  I did a search with \[i\][a-zA-Z0-9 +*-]{1,20}\[j\];

[fractalsbyx]

Update the regression test outputs, and this will be good to go.
(e.g. tests/regression_tests/allen_cahn_explicit/gold_output.txt)

[jijn]

gold_output.txt updated