View2D - more consistency and configuration flexibility

Currently, if you want to set maxZoom or maxResolution or zoomFactor in the View2D options you actually have to set the three of them. This PR fixes that.

This PR also removes the numZoomLevels option, replacing it by maxZoom. This is to be consistent with ol.source.XYZ and ol.tilegrid.XYZ, where maxZoom is used.