feat: adds the Size() method to the nmt API

[staheri14]

Overview

Closes #160

Checklist

• New and updated code has appropriate documentation
• New and updated code has new and/or updated testing
• Required CI checks are passing
• Visual proof for any user facing features like CLI or documentation updates
• Linked issues closed with keywords

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@d0789c6). Click here to learn what that means.
The diff coverage is 100.00%.

❗ Current head ab2d666 differs from pull request most recent head b78ba42. Consider uploading reports for the commit b78ba42 to get more accurate results

@@            Coverage Diff            @@
##             master     #191   +/-   ##
=========================================
  Coverage          ?   95.71%           
=========================================
  Files             ?        5           
  Lines             ?      560           
  Branches          ?        0           
=========================================
  Hits              ?      536           
  Misses            ?       14           
  Partials          ?       10           

Impacted Files	Coverage Δ
nmt.go	98.02% <100.00%> (ø)