Setting up testing for the repository

[fnattino]

I think the repository should include enough data not to make queries to the Overpass API while running continuous integration. We should probably be storing the data in a form that is as close as possible to the way in which raw (OSM) data is provided, in order to test functionality of the cleaning steps. We should also consider storing cleaned data later on (e.g. cleaned street network), in order to test the functionalities of the steps that follows.

• add test data to the repository for realistic testing (considering Bucharest, our study case for the moment).
• include the script that downloads the data, in order to be able to update the test datasets in future.
• tests running Overpass API queries should be skipped unless they are specifically flagged to run (EDIT: there is a nice testthat functionality which suppresses tests on CI: skip_on_ci ) - include example of a test that needs to be skipped.
• all other tests should instead run using the testdata - include an example of this too.

Closes #12.

[cforgaci]

Looks good, thanks! I left a couple of comments on the use of functions.