[dashing] IMU driver improvements

[hidmic]

This pull request introduces a number of improvements, namely:

• Refines covariance defaults for most VN100 outputs.
• Makes VN100 attitude estimation frame configurable.
• Exposes VN100 on-board Hard/Soft Iron calibration settings as parameters.
• Exposes VN100 magnetic and gravity models settings as parameters.

As such, I'd rather not squash commits in this PR.

[hidmic]

Tagging @clalancette for awareness.

[hidmic]

@versatran01 friendly ping.