feat: extend mature miRNA annotations

[deliaBlue]

Closes #65

In this PR the changes applied are:

• Creation of a class that extends mature miRNA coordinates
• Implement a script that uses the class
• Create unit tests for both the class and the script
• Update dev dependencies to include gffutils and the pytest and coverage

[uniqueg]

Getting there. 3 more minor comments, I think after we can merge :)

[deliaBlue]

If I am not wrong, we said that the extension would be for the user to define; provide this option on the config file has not yet been implemented. Do you still want the extension to be for the user to change or you rather leave it at 6 by the moment?

[uniqueg]

If I am not wrong, we said that the extension would be for the user to define; provide this option on the config file has not yet been implemented. Do you still want the extension to be for the user to change or you rather leave it at 6 by the moment?

Yes, it should indeed by up for the user, good that you remembered. Please add it to the config file; this PR is the right place to do this. Rest looks good now.

[deliaBlue]

While adding the extension option I saw that config_schema.json and config_template.yaml are outdated. They need to have removed the "isomiRs generation parameters" and in case of the template, it need to have the addition of the extension parameter. Can I do a simple PR named "build: update config files" and do these two changes?