Refactor POD and add inverse_transform

Types of changes

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• Bug fix (non-breaking change which fixes an issue),
• New feature (non-breaking change which adds functionality),
• Breaking change (fix or feature that would cause existing functionality to change),
• Documentation.

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• I have checked there is no similar merge request on the track,
• I have read the CONTRIBUTING document.
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• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

Description

Simplify the usage of POD by removing usage of Space. Also add inverse_transform method.

Closes #132 (closed)