Include Ensembl Tark API for reference retrieval

[XLIU-hub]

This pull request allows for references retrieval of older Ensembl versions. It resolves #16.

Currently, Mutalyzer uses GRCh38 and GRCh37 APIs, which only support reference ID queries with newest versions. In this PR, we include a new API, Ensembl Tark, which also supports for queries with older versions.

Note that Tark supports only human transcripts. In the future, it could be extended to genes and other organisms.

Given that Tark provides only exon sequences in the response, we make use of the previous Ensembl REST APIs to retrieve the entire transcripts sequences (introns included).

Changes

• Add reference source options for ensembl in command line interface (users can choose between ensembl, ensembl_rest and ensembl_tark).
• Convert ensembl_tark JSON-type output into the retrieval model.
• Add tests to check that the new functionality works for ensembl, ensembl_rest and ensembl_tark.