feature: support for import and export datapackage and tableschema

The [datapackage model](https://github.com/frictionlessdata/datapackage) by [frictionlessdata](https://frictionlessdata.io/) is a schema to describe datasets and their [tableschema](https://datapackage.org/standard/table-schema/). The conventions seems adopted by some implementation of the [RO-Crate](https://www.researchobject.org/ro-crate/) community, a sidecar concept very similar to pygeometa.

Would be nice to support reading and writing of the datapackage and tableschema conventions.

There are also python libraries to work with [datapackage](https://github.com/frictionlessdata/datapackage-py) and [tableschema](https://github.com/frictionlessdata/tableschema-py)

Start by reading the linked Data Package and Table Schema standards, then review the referenced Python libraries to understand the required conventions. Identify pygeometa's metadata read/write entry points and define what complete support means for both importing and exporting these formats.

Written by the indexing model from the issue text.