Fix 6D Settings Parsing for Versions Below 1.21

[OliverGlandberger]

TLDR

Read6DOFSettings always failed against the pre-1.21 XML format, so clients pinned below 1.21 (e.g. QTM Connect for MATLAB at 1.17) got no 6DOF bodies. Regression from the tinyxml2 rewrite (#109).

Problem

In the legacy branch of Deserialize6DOFSettings:

• Bodies and Body were looked up on the document root instead of inside The_6D (returned false immediately)
• TryReadSetPointsOld iterated Bone elements (previously Point)
• TryReadSetEuler ran once per body and treated a missing <Euler> as an error
• dataAvailable was only set when at least one body was found

Solution

• Update the lookups to use The_6D, read Point elements, read Euler once after the body loop and treat it as optional (complete or absent)
• Set dataAvailable for the element rather than per body
• Read6DOFSettings now sets an error string naming the protocol version on parse failure

Tests Run

• Ran full test suite
• Verified end to end against QTM with QTM Connect for MATLAB at 1.17

[OliverGlandberger]

@Capelliexp I was hesitant to add the tests because I think they are likely overkill. I kept them for the initial draft, to give you a chance to weigh in.

[OliverGlandberger]

Spoke with @Capelliexp and we're ignoring the windows build failure caused by doctest and is unrelated to this change.