Move gridding test, separate data

[deeplycloudy]

The test in examples/grid/grid_sample_data.py should be moved into the test directory, which requires making the gridding script discoverable by the tests - perhaps install to bin?

At the same time, separate out the data from the test directory, and rewrite the sample data discovery method in test.common. Per py.test docs, "You shouldn’t have init.py files in your test directories."