Move ntv2reader module tests

[harry093]

At the bottom of the ntv2reader module there is a test. This should be moved to a file in the geodepy/tests/ directory.

The existing test has a hardcoded path in it. Should this be removed?

[BatchelorJ]

I hadn't noticed this, that's really great! We could move this to a new test module, download the file from the ICSM S3 Bucket to a temporary directory using the tempfile module (see test_surveyconvert module), then run the tests using the temporary copy. Not sure of the impact on performance of downloading a 236 MB file each time the tests are run, but it's an option. Another would be to get Jack to create a small test grid (say 20 x 20 nodes) that we could put directly in the test suite.