SeqFeature cleanup

I am finding code to parse GenBank-style location strings into `SeqFeature` objects in multiple places in Biopython:

- In `Bio/SwissProt/__init__.py`, in the `_read_ft` function;
- In `Bio/SeqIO/SwissIO.py`, in the `_make_position` function;
- In `Bio/GenBank/__init__.py`, in the `_pos` function.

Note that all three functions are private.

It would be better to replace these three functions by one public function, maybe `SeqFeature.fromstring`.