Common structure for statistical test functions

[TuomasBorman]

All functions that perform statistical tests (we will also have plotting functions in the future) should follow the same structure:

1. Check input: verify that the input data is in the correct format.
2. Extract data: output data as a long-formatted table.
3. Run analysis: perform the statistical test.
4. Wrangle output: format results consistently (details to be agreed in the future).
5. Output results: return results in a standard format.
6. Add results to (Tree)SE: in add* functions, add the results to the appropriate slot (to be decided).

By using the same structure, we avoid extra work and keep things as simple as possible. This also ensures that new tests are easy to implement in the future.

Each of the steps above will be discussed in separate issues.