Simplified resource configuration

This PR:

1. Removes data_repository from site.cfg and ```iris.config.DATA_REPOSITORY```
2. Removes master_data_repository from site.cfg and ```iris.config.MASTER_DATA_REPOSITORY```
3. Adds test_data_dir to site.cfg and ```iris.config.TEST_DATA_DIR```
4. Removes the ```iris.io.select_data_path()``` function.

It is intended to simplify the setup and configuration for users who wish to run the unit tests and attempts to make the test data setup similar to the sample data setup.