fix: transcript sampler

[balajtimate]

Description

• Refactored transcript-sampler to output the representative transcript from the given gtf annotation file, and return the Poisson-sampled number of transcripts, based on the given expression csv as well as the representative transcript for each gene.
• Updated test files for transcript-sampler

Fixes #14

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

Checklist

Please carefully read these items and tick them off if the statements are true
or do not apply.

• I have performed a self-review of my own code
• My code follows the existing coding style, lints and generates no new
  warnings
• I have added type annotations to all function/method signatures, and I
  have added type annotations for any local variables that are non-trivial,
  potentially ambiguous or might otherwise benefit from explicit typing.
• I have commented my code in hard-to-understand areas
• I have added ["Google-style docstrings"] to all new modules, classes,
  methods/functions or updated previously existing ones
• I have added tests that prove my fix is effective or that my feature
  works
• New and existing unit tests pass locally with my changes and I have not
  reduced the code coverage relative to the previous state
• I have updated any sections of the app's documentation that are affected
  by the proposed changes

If for some reason you are unable to tick off all boxes, please leave a
comment explaining the issue you are facing so that we can work on it
together.