ts_prioritize in merge.py

[water-e]

Idea is to have a function that prioritizes time series in order given like ts_merge. The listed series would be used from their first_valid_index to last_valid_index, but no filling of internal nans. Could be a switch to ts merge. The implementation could be as simple as filling the (copied) series and filling internal (first_valid to last_valid) nans with a weird value like -1.234e16, merging, and replacing of the weird values. Should be added to the merging tutorial notebook as well.