Easier Indexing / Subsetting of KData

[JoHa0811]

So far, in order to index the data part of kdata, one has to access it via kdata.data[index]
It would be nice if one could simply state kdata[index].
Also, subsetting kdata is relatively tedious.
A perfect case would be if one could subset kdata by usng kdata[subsetting_indices].

Not sure if these are possible right away.

[fzimmermann89]

#243

I have some ideas about this and could take it at some time.
currently, it is not possible and needs some backend work to properly do it.