Vertical: improve automatic guess of item selection

If "z" is not the first item we have to specify both z_item and item in creation of VerticlModel or VerticalObservation. With only 2 items it should be enough to specify only one of them.

reason: z_item is default to pos 0 and then:
https://github.com/DHI/modelskill/blob/ea4c9e325cac2b22fc70c1023fdf5d7d62e3e731/src/modelskill/timeseries/_vertical.py#L50-L59)

if we set item=0, we get after _get_name()
item = "v"
but z_item is also 0 and we be "v" as well.

if we instead set z_item=1, we get
item = 1 = "z"
z_item = 1 = "z"

With only two items it should be enough setting one explicitly