API for importing data from any support.

This API defines a secure import flow, that allows to import data in a container, then checks the container for data consistency and finally process data and appended them to `GraphAPI`.

See `org.gephi.io.importer.spi` package for defining new importers and `org.gephi.io.processor.spi` for processors. Processors come after importers and are responsible for appending import results.

Import tasks can also carefully log what they do and where some problems where found. The report can then be displayed to users.