Add data_loader sub-package

[aphearin]

@alexalar this PR brings in a new data_loaders sub-package with the basic features we discussed today during group meeting. A few things to note:

1. I modified diffstarpop/.coveragerc so that code inside data_loaders will not count towards the coverage fraction
2. There is one unit test that enforces that dummy_data_loader imports. This unit test will run in our CI.
3. There is a separate unit test that will only run on a machine where the data exists.