Updated docs and added method in core.py

[elenikiachaki]

This PR addresses #274.

The printed outputs shown in tutorial.md were not consistent with the actual outputs I got when running tutorial.py, so I updated them. I’m not sure whether the documented outputs were outdated, came from a different protein/example, or if there is an issue in the current code and I am not getting the same results..

I also noticed that some print examples shown in tutorial.md were not actually printed by tutorial.py, so I added the corresponding print commands to the script.

Examples of outputs I could not reproduce and I am not sure where they are coming from:

>>> seq.positions[45].properties
{'ptm_glycosylation': <PositionProperty Glycosylation Site from GlyGen>}

>>> seq.positions[45].properties['ptm_glycosylation'].metadata
{'subtypes': ['N-GlcNAc', 'O-GalNAc']}

>>> seq.positions[10].properties['mobidb_disorder_propensity']
<StructuralDisorder, Structured, observed>