Expand source and destination paths

[d33bs]

Description

This PR adds capabilities to CytoTable for source and destination path expansion (using ~ and environment variable replacements) and resolution (using . and .. to navigate an absolute path). This work comes up as the result of collaborations with @shntnu .

Closes #88

What is the nature of your change?

• Bug fix (fixes an issue).
• Enhancement (adds functionality).
• Breaking change (fix or feature that would cause existing functionality to not work as expected).
• This change requires a documentation update.

Checklist

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• I have added tests that prove my fix is effective or that my feature works.
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[falquaddoomi]

Nice work! I imagine being able to use variables and relative references will be appreciated by your users. I left a few comments, but I didn't see anything of enough concern not to approve it.

[d33bs]

Thank you @gwaybio and @falquaddoomi for your reviews! Merging after addressing your comments.