Make datetime input more flexible for az_hourly()

[Aariq]

Addresses #44 to allow dates alone to be passed to az_hourly(). When a date is passed to end_date_time the results contain data through the end of that date.

This one was tricky because while the AZMet API uses 23:59:59 for midnight rather than 00:00:00, it considers the time interval between 23:00:00 and 23:59:59 exactly one hour. This has made the parse_params() function need to be fairly complicated. Would be happy to simplify things if there are any obvious ways to do so.

The data retrieval functions now print messages whenever parse_params() is "filling in gaps" in inputs. E.g. start and end not supplied, end not supplied, or end not supplied to hour precision (for datetimes).

Other changes:

• Removes API mocking from most tests—with all the tests using the current date and time added, the mock API requests no longer were useful in many cases. I'll fully remove this infrastructure and instead skip all tests that hit the API on CRAN and if the AZMet API is down in a separate PR.

[jeremylweiss]

Other than my comment on az_hourly.R, the function works as expected. The changes to datetime input offer a nice added convenience.

[jeremylweiss]

I'm glad you caught the timezone detail! Also, I like the user feedback messages, which I'm learning are important when it comes to UI-UX-web-data-viz work.

[Aariq]

Ok! I think this is finally ready for another round of review. Again, you can safely ignore all the changes in .json files in tests/testthat/—these are the mocked API responses.