Add some tests for the date handling logic

[xoen]

Added some tests for the internal utility functions used to handle dates:

• to_tuples()
• normalise_dates()

These would have prevented this glitch earlier :)

I've also made some small refactoring after introducing these tests:

• to_tuples() just takes the Vec<Data> instead of the whole RegionData struct (as it's only using the data anyway)
  • this also simplify the test, see current version vs. original iteration where the test had to construct a RegionData even tho the logic only involves the data
• to_tuples() uses Rust's Iterators and avoids mut/Vec::push()

This should resolve Issue #14

[xoen]

I know it's kinda cosmetic but maybe it's worth deciding on what to do in regards of the American vs. British spelling before introducing more inconsistencies :)

For example here I've named one of the tests deserialize_power_data_test() with a z 😬

[xoen]

FYI: I've tweaked the spelling.

[jnioche]

thanks @xoen