feat(Execution workflow): APA-Scan

## WHAT
**Write** execution workflow for [APA-Scan](https://github.com/compbiolabucf/APA-Scan). Use the provided [small files for testing](https://github.com/iRNA-COSI/APAeval/tree/main/tests/test_data) (running the workflow on real data is a different issue).

## CHECKLIST
* [ ] Use [snakemake template](https://github.com/iRNA-COSI/APAeval/tree/main/docs/templates/snakemake) or [nextflow template](https://github.com/iRNA-COSI/APAeval/tree/main/docs/templates/nextflow) to create your workflow.
* [ ] Comment your code
* [ ] Run individual rules/processes in either conda envs or docker/singularity containers for reproducibility
* [ ] Input: .bam or .fastq from [test_data]((https://github.com/iRNA-COSI/APAeval/tree/main/tests/test_data))
* [ ] Give [feedback about the method](https://forms.gle/mAAkbRJqhedJnSeJ6)

## OUTPUTS (see [specification](https://github.com/iRNA-COSI/APAeval/blob/main/execution_workflows/execution_output_specification.md)):
* [ ] Output: Adhere to output specification for Identification challenge
> This BED file contains single-nucleotide position of poly(A) sites identified by the tool.
Fields:

> chrom - the name of the chromosome
chromStart - the starting position of the feature in the chromosome
chromEnd - the ending position of the feature in the chromosome; as identified PAS are single-nucleotide, the ending position is the same as starting position
name - defines the name of the identified poly(A) site
score - not used, leave as "."
strand - defines the strand; either "." (=no strand) or "+" or "-".

* [ ] Output: Adhere to output specification for quantification challenge
> This BED file contains positions of unique poly(A) sites with TPM values for each identified site in the score column.

> chrom - the name of the chromosome
chromStart - the starting position of the feature in the chromosome
chromEnd - the ending position of the feature in the chromosome; as identified PAS are single-nucleotide, the ending position is the same as starting position
name - defines the name of the identified poly(A) site
score - TPM value for the identified site
strand - defines the strand; either "." (=no strand) or "+" or "-".

* [ ] Can APA-Scan do differential PAS quantification? If yes:
* [ ] Output: Adhere to output specification for differential usage challenge
> This TSV file contains two columns:

> - gene ID
> - significance of differential PAS usage

> Column names should not be added to the file.