Quick boundary fix

[dalmijn]

Issue addressed

Fixes #429

Explanation

See issue #429

Checklist

• Branch is up to date with master
• Tests & pre-commit hooks pass
• Updated documentation if needed
• Updated changelog.md if needed

[verseve]

Thanks for fixing this issue quickly, @dalmijn !
To improve readability a bit I assigned lateral.subsurface.flow.constanthead to constanthead and lateral.subsurface.flow.aquifer to aquifer. Also updated the changelog.

[dalmijn]

@verseve, thanks for adding it to the changelog!
Improvement of readability looks very logical/ good.
Shall I then merge it?

[verseve]

@verseve, thanks for adding it to the changelog! Improvement of readability looks very logical/ good. Shall I then merge it?

Yes, the honor is yours :-)