Add in test environment connections

[rmbielby]

Brief overview of changes

This could get quite big. We need to add in the test environment examples, but also the environment needs to follow through appropriately in all the different components. And it'll be worth doing the same with api_version and dataset_version whilst we're on it.

Why are these changes being made?

We need eesyapi to work across multiple environments: dev, test, preprod, prod

Detailed description of changes

Environment and API version

• Switched environment to ees_environment to avoid potential confusion with base R environment.
• Cascaded ees_environment and api_version through the function hierarchy:
  • e.g. query_dataset() -> post_dataset() -> api_url()
• I've set all these to default to NULL until api_url(), at which point it will choose "test" if nothing's been set by the user on ees_environment and "1.0" if nothing's been set on api_version. Once we're working on prod I think we reverse that, so that all functions explicitly default to "prod", but the current set up just means we should be able to switch between dev, test, preprod and prod a little more easily where needed.

example_id()

• Renamed default example set to "absence"
• Test added as an option on attendance and absence (and kept dev)

Function examples

Some function doc examples were hardwired with dev based codes. I've updated most of these to use example_id() instead to help make it a little easier to switch between platforms whilst we're developing.

Issue ticket number/s and link

Resolves #39

[cjrace]

@rmbielby - test data sets are now published and can be viewed in the test data catalogue

[cjrace]

Not my most concise review!

Commented with some ideas that could be put into future issues, plus a handful minor of things to actually address in this PR

[cjrace]

All good to merge!