Improve ergonomics

[ielis]

Improve ergonomics of working with phenopacket data.

Closes #86

[iimpulse]

Was there a reason to not leverage phenopackets python library? It has serialize deserialize using protobuf

[ielis]

@pnrobinson the PR is ready to review.

These are the highlights:

• pytest fixtures show how to create a phenopacket programatically
• Phenopacket & friends can be read from and written to JSON or protobuf. The loading will raise a ValueError if a required field is missing, per Phenopacket Schema. Sadly, only the first failure is reported at the moment (we can improve later)
• we have type hints for the phenopacket components

We can import all classes as:

from pyphetools.pp.v202 import *

Please give me a :+1: if looks good and let me know if you find anything odd. Thanks!