add yaml interface

[dbrakenhoff]

Add option to read/write Pastas models to/from YAML files using PastaStore.

• initialize yaml interface class in pastastore init
• add PastastoreYAML class
• includes yaml read/write methods
• add example notebook
• add tests
• add example dataset + extra example data

For more information and background see notebook here

[OnnoEbbens]

Great addition!