some stub unit tests for Dataset

Add some infrastructure and a couple of stub tests for Dataset. The
infrastructure is basically updating the ``generate-dem-chunks.sh`` tool to
also dump a ruaumoko-format dataset as well as the zip files for testing the
downloader. This dataset is then used to lightly test the Dataset code.

The Dataset code is updated to know about configurable tile sizes. Currently
this is not tested by the test suite. It'll probably be tested by manually
verifying a few elevations and hard-coding them into the test suite.