Add a freshness parameter

[tpoisot]

What the pull request does
Fixes #23 by warning users if the taxonomy is over 30 days old (give or take a few hours).

Type of change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ❇️ New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📖 This change requires a documentation update

Checklist

• The changes are documented
  • The docstrings of the different functions describe the arguments, purpose, and behavior
  • There are examples in the documentation website
• The changes are tested
• The changes do not modify the behavior of the previously existing functions
  • If they do, please explain why and how in the introduction paragraph
• The Project.toml field version has been updated

[codecov-io]

Codecov Report

Merging #24 (a525299) into main (eabb3d1) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main       #24   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            7         7           
  Lines           75        77    +2     
=========================================
+ Hits            75        77    +2     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/NCBITaxonomy.jl	100.00% <100.00%> (ø)

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