add a hook to validate the collection at the end of import_directory

[jpmckinney]

In order to run uniqueness validations, all the data needs to be imported into the database. If import_directory can call a validation method (which can be a noop in BaseImporter) before returning, I can override that method to issue warnings if collection/table-level validations don't pass.

Right now, I need to run these validations from outside Pupa, which means I don't see the validation errors when I run pupa update while developing the scraper.

[jamesturk]

this should be easy to add in general- where would your override live though?

[jpmckinney]

I'd probably import the appropriate Importer class and override the validation method with my logic. I don't know common Python patterns for injecting code/behavior into an existing class, so there are likely better ways.

[jpmckinney]

Are there any Python patterns for injecting code into, for example, PersonImporter's postimport?