tests: remove DownloadedDatasetTestCase

Since this TestCase sub-class merely set a constant, it seems more
idiomatic to have, well, a constant. Fix up uses of
DownloadedDatasetTestCase elsewhere.
api: expose expected tile resolution in app config

Add a new configuration parameter, ELEVATION_TILE_RESOLUTION, which can
be used to configure the expected tile resolution. Required for the test
suite in which the mock dataset has a different resolution.
tests: add test for web api

Using the wonderful Flask-Testing package, exercise the web API a little
checking for failure modes and that it gives outputs consistent with
calling Dataset.get() directly. The calls to Dataset.get() are
indirectly tested via the ASCII art maps test.
Merge pull request #23 from rjw57/test-api

add tests for the web API