Proposed 'tools' module restructuring

[mpsonntag]

The current tools module structure does not allow to put classes at the module level to make them easier to import and use. See #317. Linking the FormatConverter or the VersionConverter class, currently leads to an import cycle.

By creating a submodule odml.tools.converters and move both format_converter.py and version_converter.py into the new submodule the described import cycle can be avoided and all Parser and Converter classes can be made available via the respective __init__.py files.

[mpsonntag]

As an additional note: this change of structure will lead to a required update of odml-ui since it is importing the VersionConverter.

[mpsonntag]

Closed via PR #342