Flowserv-core

* Make `path` argument for `copy_files` less restrictive (\96)

* Fix issue with `S3Volume.walk` return value (\95)

* Reset hard-coded workflow repository URL to point to the master branch.

* Add option to resolve references to external files when reading Yaml files.
* Support inclusion and import of files in instructions markdown text.

* Make `BucketStore` generic for storing workflow files in different types of cloud storage buckets (\80).
* Add Google Cloud File Storage bucket implementation (\80).
* Support different storage volumes for different types of workers.
* Add support for workflow steps that are implemented as Jupyter Notebooks (\79).
* Include flowapp to run workflow templates using streamlit GUI.

* Fix issue when creating database object on Windows.
* Fix issue with schema validator on Windows.
* Fix issue with bucket store unit tests on Windows.
* Fix unit test issues for Windows.
* Fix issue with subprocess worker on Windows.