Refactor handling of output fastq file handles

[b97pla]

• Refactored handling of file handles for output fastq files in order to make mainly the demux-module classes more single-purpose.
• In the process, the Demultiplex class could be eliminated and the FastqFileWriterHandler class added
• Updated the test suite accordingly

[codecov[bot]]

Codecov Report

Merging #4 into master will not change coverage. The diff coverage is 100%.

@@          Coverage Diff          @@
##           master     #4   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           5      5           
  Lines         161    159    -2     
=====================================
- Hits          161    159    -2     

Impacted Files	Coverage Δ
fastq_demux/demux.py	100% <100%> (ø)
fastq_demux/fastq_demux.py	100% <100%> (ø)
fastq_demux/writer.py	100% <100%> (ø)

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