Bw-processing

* Fix [6 - `del_resource` should support resource groups](https://github.com/brightway-lca/bw_processing/issues/6)
* Fix [7 - Error out when adding existing resource to package](https://github.com/brightway-lca/bw_processing/issues/7)
* Fix [9 - Multiple readings of same proxied data resource in FilteredDataPackage causes errors](https://github.com/brightway-lca/bw_processing/issues/9)
* Fix [10 - Add function for filtering resource groups](https://github.com/brightway-lca/bw_processing/issues/10)

* Keep package indexers when creating a `FilteredDatapackage`.

Nearly complete reconceptualization of the package structure and logic, based around the use of [PyFilesystem2](https://docs.pyfilesystem.org/en/latest/). Much more complete package, with better testing and documentation.

Small background improvements to package creation

Remove `bw_projects` as a dependency

First public release