Factor out fixtures into base class
Add tests for tide-levels `interval` parameter

- default to 1 (minute) if not given
- must be integer
- must be between 1 and 60 inclusive

[#74785098]
Implement `interval` parameter to `tide-levels`

- Return a 400 if it's not a valid integer, or if it's outside a valid
  range
- Return every interval'th tide level

https://www.pivotaltracker.com/story/show/74785098
[Delivers #74785098]
Merge pull request #42 from sealevelresearch/interval-parameter

Implement `interval` parameter to `tide-levels`