Improve NormalizeData

[vrtulka23]

Initialize NormalizeData with data columns

with snt.NormalizeData(xaxis=True, yaxis=True, xdata, ydata) as n:

If zdata is a scalar, following error ocures:

    zminpos = np.nanmin(vdata[vdata>0]) if np.sum(vdata>0) else np.nan
TypeError: '>' not supported between instances of 'list' and 'int'

Ideally, one would set x and y first and then the z data values