added tests against slocum and seaglider netcdfs

[callumrollo]

First step to resolving Issue #19

These tests download previously created netcdfs from the slocum and seaexlorer examples, and check that both the data and metadata remain unchanged.

These tests should pass for refactoring. If intentional changes are made to the resultant netcdfs, we can update the examples we are testing against

[jklymak]

This looks great. I guess we need to have the testdata in the repo though, rather than grabbing via url?

[jklymak]

Suggest moving the comparison data to tests/results/ or some such. A separate repo isn't the end of the world, but how do we know what yo used to make the test data? This way we can see if the test data changes.

[callumrollo]

Cool, I'll add the test data into tests/results they're both < 1MB netcdfs. That will also make the test code simpler. If storing the versions becomes a space issue later we'll figure something out