fix: allowed LatLngBounds.center to work across world boundary & added simpleCenter

[monsieurtanuki]

What

• The formula referenced for the LatLngBounds.center mentioned a [-180, +180] fix - now that fix is implemented.
• A more intuitive LatLngBounds.simpleCenter is now implemented.

Impacted files

• latlng_bounds.dart: fixed the computation of center according to referenced link; added a more intuitive simpleCenter method.
• latlng_bounds_test.dart: added the tests that failed in the initial issue

[JaffaKetchup]

@josxha Did you have any other concerns about this?

One thing maybe we should consider is writing a small doc snippet to compare them and which is better for what situation.

[JaffaKetchup]

LGTM, although maybe a little bit of better documentation would be good? Maybe a diagram of where the two centers would calculate different values, and which is correct to use for which situation?
Will wait for @josxha to approve before merge.

[josxha]

The changes in this pull request are straight forward. The only two things I struggle with are the name simpleCenter. I'd prefer a more descriptive way but haven't found one ever since. And the other thing is that we should use this new center calculation for the camera, shouldn't we?

[monsieurtanuki]

Regarding the naming: in the original code transformed here into dart, the description of the "complex" center is

Midpoint - This is the half-way point along a great circle path between the two points".

[josxha]

Alright 👍