Leave config file only for general importers

[lorenaqendro]

Fixes #420

Description

Context: When creating the general csv importer, there was a need for a configuration file where to specify how to interpret the dataset and other info so it could be used in the DC data structure (see example). At the time, we decided that it made sense to have a configuration file for each importer not only the general ones. This way we could leave to the user the decision of personalising the built-in importer via the config file. This meant of course changes in the present importers we did not implement. What seemed to be a good design decision at the time (might still be) was not adopted in practice.

In this PR I left the config file only for the general importers and removed it from the rest. I created an Abstract class to represent the importers that have config files so it should be ok if we want it for future importers.

Checklist

• []Created new test(s) (if applicable)
• [ ]Updated the README / documentation (if applicable)

[arya-hemanshu]

Looks good, just one question, why creating another Abstract class rather adding another constructor to already existing Abstract class?

[lorenaqendro]

@arya-hemanshu I did it so we can create a kind of hierarchy for the general importers or better the ones that provide a config file. This is will be more useful when we have other general importers of course.

[arya-hemanshu]

Good to merge