Use logger package

[pkrog]

Replace custom logger system by an existing logger package if possible.

[pkrog]

• futile.logger.
• lgr. R6.
• log4r.
• logger.
• logging.

[pkrog]

• [x] Implement a get_logger() global function that returns the "biodb" main logger. Or use the root logger lgr::lgr ==> No, good practice is to create one logger for each package.
• [ ] In each instance of Biodb class, create a logger that is the son of the main logger.
• [ ] Do not forget to destroy Biodb instance's logger inside terminate().
• [ ] Add a logger in each connector instance?

[pkrog]

For progress messages, use progress package.

• [ ] Add an option to turn off progress messages.
• [x] But also warn observers with specific messages, thus allowing tests on Travis-CI to do not fail due to stalling.
• [x] To do this we create a class Progress that calls observers (we pass list of observers to the constructor) and use progress package to display progress messages (we pass the progress message to the constructor).