Increase tolerance of surge sanity check

From +/- 1.0 metres to +/- 2.0 metres
Merge pull request #3 from sealevelresearch/increase-surge-tolerance

Increase tolerance of surge sanity check
BODC parser shouldn't filter "improbable" values

We'll decide what's improbable!
Add missing POLTIPS parser tests

They got missed off the previous pull request.