Allow passing roots to xcube.webapi.viewer.Viewer

[forman]

The class xcube.webapi.viewer.Viewer now accepts root paths or URLs that will each be scanned for datasets. The roots are passed as keyword argument roots whose value is a path or URL or an iterable of paths or URLs. A new keyword argument max_depth defines the maximum subdirectory depths used to search for datasets in case roots is given. It defaults to 1.

Closes #990

Checklist:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/source/*
• Changes documented in CHANGES.md
• GitHub CI passes
• AppVeyor CI passes
• Test coverage remains or increases (target 100%)

[TejasMorbagal]

Functionality works. The feature can be handy for viewing smaller datasets, but for larger once loading of datasets is slower, which is expected.

[forman]

Thanks!