Add LocalDateRange::toPeriod

[BenMorel]

Following #48, in addition to LocalDateRange::getDuration, it would be useful to add a LocalDateRange::toPeriod() method.

There are several ways to implement this method. Take for example 2021-03-28/2022-04-01, we can represent it as either:

• 1 year, 1 month, -27 days
• 1 year, 0 month, 4 days
• 0 year, 0 month, 369 days

Not sure which one we should choose here. The first one has a negative part for a non-negative range, so is probably not that intuitive; the third one is probably not what you would expect either, and one may argue that if you just want a raw number of days, you may use getDuration() instead. So at first glance, I'd go with the second option.

Another solution could be to offer several methods, but this can add to confusion.

I can see that Java's threeten-extra has a [toPeriod](https://www.threeten.org/threeten-extra/apidocs/org.threeten.extra/org/threeten/extra/LocalDateRange.html#toPeriod()) method, it would be nice to see which way they followed.

Cc @solodkiy

[solodkiy]

I am not really fan of this Period concept. Never used it in my production. I personally could imagine use of convert 2021-03-28/2022-04-01 to Period 369 days, but 1 year, 0 month, 4 days.. what should we use it for?

[BenMorel]

Period is definitely useful to represent something like +6 months, that you cannot represent with a Duration, that only works with days.

In the case of LocalDateRange::toPeriod(), one use case I can think of is for display purposes, for example to display a person's age, in which case I think 1 year and 4 days is what you would expect here.

[BenMorel]

Just tried it on Java, the output of the example above is P1Y4D, that is, 1 year and 4 days :+1:

And the implementation just uses Period::between(), which we have already implemented, so the implementation will be trivial.

[BenMorel]

Implemented in 213a4ba7184d983dfe2087658b68eea7140d009f.