[ENH] Add to_fasta method to janitor.biology

[naseem173]

PR Description

Please describe the changes proposed in the pull request:

• Add a new to_fasta convenience method to janitor.biology that writes the identifier and sequence columns of a DataFrame out to a FASTA file. This is the write-side counterpart to the existing join_fasta (which only reads a FASTA file into a DataFrame column).
• Add a round-trip test (tests/biology/test_to_fasta.py) that writes sequences out with to_fasta and reads them back in to confirm the written FASTA file matches the source data.
• Add a CHANGELOG.md entry under [Unreleased].

This PR resolves #646.

PR Checklist

Please ensure that you have done the following:

1. PR in from a fork off your branch. Do not PR from <your_username>:dev, but rather from <your_username>:<feature-branch_name>.

2. Add a line to CHANGELOG.md under the latest version header (i.e. the one that is "on deck") describing the contribution.
   • Do use some discretion here; if there are multiple PRs that are related, keep them in a single line.

Automatic checks

There will be automatic checks run on the PR. These include:

• Building a preview of the docs on Netlify
• Automatically linting the code
• Making sure the code is documented
• Making sure that all tests are passed
• Making sure that code coverage doesn't go down.

Relevant Reviewers

Please tag maintainers to review.

• @ericmjl

[codecov]

Codecov Report

❌ Patch coverage is 28.57143% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.94%. Comparing base (901f4b3) to head (e031502).
⚠️ Report is 218 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #1701      +/-   ##
==========================================
- Coverage   87.56%   85.94%   -1.62%     
==========================================
  Files          95      125      +30     
  Lines        6819     9968    +3149     
==========================================
+ Hits         5971     8567    +2596     
- Misses        848     1401     +553     

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.