Consistent naming across all state estimators

[vegard-solum-4ss]

This PR is related to user story DLAB-49

Description

Rename attitude to euler to ensure consistent naming across ass state estimator classes.

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[vegard-solum-4ss]

Should we also implement the error and bias as methods to align with the other states? Now they are implemented as properties.

[vegard-solum-4ss]

Should we also implement the error and bias as methods to align with the other states? Now they are implemented as properties.

I updated these to be methods. Let me know if we should change it back.