27 add dataframe iteration to the pandas plugin

[xaviernogueira]

Changes:

• I combined all your pandas validator tests into one .py file.
• New .iterate() function in the BaseValidator baseclass.
• Implementation of overrides for df.pydantic.itertuples(schema) and df.pydantic.iterrows(schema) where the behave as normal except non-schema validated rows are skipped (silently or not, depending on verbose).
• Implementation of df.pydantic.iterschemas() which returns the schema models.

Closes #27

Let's merge into PR #23 when this is ready @wesselhuising