Bug fix for boundary handling

[einola]

Pull Request Title

Task List

• Defined the tests that specify a complete and functioning change (It may help to create a design specification & test specification)
• Implemented the source code change that satisfies the tests
• Documented the feature by providing worked example
• Updated the README or other documentation
• Completed the pre-Request checklist below

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Change Description

Replaces the incorrect handling of Dirichlet boundaries in reinitnormalvelocity(). This could help improve the stability of the model, as I've seen some strange boundary behaviour before.

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Test Description

Standard tests run.

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Documentation Impact

N/A

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Other Details

None

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Pre-Request Checklist

• The requirements of this pull request are fully captured in an issue or design specification and are linked and summarised in the description of this PR
• No new warnings are generated
• The documentation has been updated (or an issue has been created to track the corresponding change)
• Methods and Tests are commented such that they can be understood without having to obtain additional context
• This PR/Issue is labelled as a bug/feature/enhancement/breaking change
• File dates have been updated to reflect modification date
• This change conforms to the conventions described in the README

[einola]

@winzerle This is the code from you we were discussing the other day. My description above is very short - but maybe it's enough? Please feel free to edit it if you'd like.

[einola]

Superceded by #797