Initial pipeline implementation

[tillywoodfield]

For each registered dataset

1. Download dataset
2. Validate dataset
3. Save valid dataset to JSON file

Closes #2

[Bjwebb]

I think ideally we'd raise a custom exception class, rather than Exception directly, but I don't think that needs to block merging this.

[tillywoodfield]

I think ideally we'd raise a custom exception class, rather than Exception directly, but I don't think that needs to block merging this.

@Bjwebb good point, I've been using Exception for the next few PRs as well, but I'll make a note to rework these into some custom exceptions when I work on the following issues which are related to error handling: #6, #9.