Previously the formula for the resolution constraint allowed going way past
the minimum zoom.
Also adjusted the center constraint to avoid a zigzag effect when going out
of resolution bounds.
This introduces a breaking change.
This options replaces the various `constrainResolution` options on interactions
and the `fit` method.
Since constraints are now the responsibility of the View, the fact that intermediary
zoom levels are allowed or not is now set in the view options.
By default, the view resolution is unconstrained.
This commit aims to simplify the computation of zoom and resolution in the
View class.
Previously zoom levels and resolution computations were mixed in different places,
ie resolution constraints, initial values, etc.
Now the View class only has the `getZoomForResolution` and `getResolutionForZoom`
methods to convert from one system to another.
Other than that, most computations use the resolution system internally.
The `constrainResolution` method also does not exist anymore, and is replaced
by `getValidResolution` and `getValidZoomLevel` public methods.
This introduces a breaking change:
The `extent` view option now constrains the whole viewport and not just the
view center.
The option `constrainOnlyCenter` was added to keep the previous behaviour.
Constraining the whole viewport and not only the view center means
that the center and resolution constraints must be applied with a knowledge of
the viewport size.
