XyData: Allow defining array(s) on construction

[sphuber]

Currently, the constructor does not allow to define any arrays to set
when constructing a new node, so one is forced to multi line code:

node = XyData()
node.set_x(np.array([1, 2]), 'name', unit')
node.set_y(np.array([3, 4]), 'name', unit')

This commit allows initialization upon construction simplifying the code
above to:

node = XyData(
    np.array([1, 2]),
    np.array([3, 4]),
    x_name='name',
    x_unit='unit',
    y_names='name',
    y_units='unit'
)

The units and names are intentionally made into keyword argument only
in order to prevent accidental swapping of values.

For backwards compatibility, it remains possible to construct an
XyData without any arrays.

[sphuber]

Any objections to this @mbercx @unkcpz ?

[unkcpz]

Sorry for the late reply. All good to me, but I didn't use this data type so can't say more.