chore: introduces a generic function to generate sample NMTs

[staheri14]

Overview

Closes https://github.com/celestiaorg/nmt/issues/132

Checklist

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

[codecov[bot]]

Codecov Report

Merging #177 (c1cf5bc) into master (1884bfb) will decrease coverage by 0.43%. The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master     #177      +/-   ##
==========================================
- Coverage   96.19%   95.76%   -0.43%     
==========================================
  Files           5        5              
  Lines         525      519       -6     
==========================================
- Hits          505      497       -8     
- Misses         12       13       +1     
- Partials        8        9       +1     

Impacted Files	Coverage Δ
proof.go	93.80% <ø> (ø)
nmt.go	97.94% <66.66%> (-1.08%)	:arrow_down:
hasher.go	97.54% <100.00%> (+0.08%)	:arrow_up:

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