Refactor I/O classes

[apriha]

Simplifies Reader and Writer classes. Adds class-based parsers (fixes #119).

[codecov-commenter]

Codecov Report

Merging #129 (4a0fe35) into develop (68ca13e) will increase coverage by 0.10%.
The diff coverage is 97.53%.

@@             Coverage Diff             @@
##           develop     #129      +/-   ##
===========================================
+ Coverage    92.87%   92.97%   +0.10%     
===========================================
  Files            8        8              
  Lines         1473     1495      +22     
  Branches       254      252       -2     
===========================================
+ Hits          1368     1390      +22     
  Misses          66       66              
  Partials        39       39              

Impacted Files	Coverage Δ
src/snps/io/writer.py	86.48% <85.71%> (-0.59%)	⬇️
src/snps/io/reader.py	97.58% <100.00%> (+0.15%)	⬆️

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[apriha]

Hi @willgdjones , @afaulconbridge , please check out these changes and let me know what you think. Thanks!