HHCODE.CENTER function returns the center of the given hhcode at the given resolution.
It will first convert the HHCode to the resolution and then compute the center and returns its lat/lon.
The function accepts 3 different types of HHCode and will return the same type for the resulting HHCode.
Since 2.6.0, the resolution is optional. In that case the length of the input is used to determine the resolution.