Override names in legend for inputs that are part of a dict

[nvaytet]

Before, when we were passing data array in a dict, but they already had names, the keys in the dict would not be used in the legend.
This meant that for example

import plopp as pp
import scipp as sc

da1 = pp.data.data1d()
da2 = da1 * 2

da1.name = "DA1"
da2.name = "DA2"

pp.plot({'a': da1, 'b': da2})

would show DA1 and DA2 in the legend, which is confusing since you would think if you pass a dict, those keys would be used. The .name on a data array may have been sat in a very different place, if fx it was inserted in a Dataset many lines above in the notebook.

Now, the keys of the dict (or DataGroup) override the underlying names on the input.
Note that the inputs' names are not modified in-place.