[STORY] Enable extension of data spaces

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When subclassing an analysis classes for a particular study, a designer may want to add another dimension to the data space (e.g. introduce a group of groups dimension). In this case, there should be a way to extend the data space and set this as the space of the subclass, while still inheriting attributes and methods from a base analysis in a sensible manner.

There are a couple of complications with this:

• enums don't support direct subclassing
• if you were to add a new dimension (say category = 0b1000), then you would want to bit-wise "and" it with the existing (non-axis) members, e.g. session 0b111 -> 0b1111, and add a new member in its place, 'matchedcategory_session' 0b0111.

Therefore, the extension should be handled by a method, e.g. DataSpace.extend(axis_name, exist_prefix), which will return a new enum with an additional axis and the values of existing non-axis members bitwise "anded" by the new axis and new members in their place with the same value prefixed by 'exist_prefix'.

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Epic: Australian-Imaging-Service/pipelines#17

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This will be handled by enforcing that frequencies are always labelled as strings. It is up to the "extended" data space to maintain the same relationships between the inherited frequencies