Create make_symlink function

[fossell]

Describe the New Feature

We noticed we do a lot of sym linking and don't have a clean way of error checking for it. Suggestion was made to create a function that can be called to check for existence and exit properly should the file not exist.

Acceptance Testing

Force errors to test the feature, e.g. no geo_em.d01.nc file present.

Time Estimate

1 day

Sub-Issues

Consider breaking the new feature down into sub-issues.

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Relevant Deadlines

??

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New Feature Checklist

• [x] Complete the issue definition above, including the Time Estimate and Funding source.
• [ ] Fork this repository or create a branch of develop. Branch name: feature_<Issue Number>/<Description>
• [ ] Complete the development and test your changes.
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[mkavulich]

Completed via #67