Overhaul Coverage based on Julia Example for Codecov

[johnzl-777]

• Instead of using the "hacked together" CI YAML, overhaul to follow example from codecov specifically for Julia (probably more robust and makes things easier to understand)
• Add codecov badge to README.md
• Restrict coverage-related steps to only occur on 8-thread test (saves time and 8-thread will also require algorithm to exercise procedure for splitting rows across multiple threads)

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (main@f96e33a). Click here to learn what that means.
The diff coverage is n/a.

❗ Current head f14fc8b differs from pull request most recent head 0a2c9c9. Consider uploading reports for the commit 0a2c9c9 to get more accurate results

@@           Coverage Diff           @@
##             main      #15   +/-   ##
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  Coverage        ?   98.55%           
=======================================
  Files           ?        2           
  Lines           ?      138           
  Branches        ?        0           
=======================================
  Hits            ?      136           
  Misses          ?        2           
  Partials        ?        0           

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