Quaternion maintenance: remove deprecated functions

[MaEtUgR]

While looking into implementing a new heading calculation for PX4/PX4-Autopilot#13535 I found some points in the quaternion implementation that I stumble over and over again. So it's time to fix them.

• The conversion functions marked with XXX DEPRECATED are there since I first saw the matrix library and are duplicate to the conversion constructors.
• I adjusted the existing conversion tests to use the constructors instead. While doing so I found some confusing reuse of qI which was set to the identity quaternion but then changed later on so I refactored the tests.
• The half angle computed for the axis angle conversion was computed using a float once instead of the template type.

[coveralls]

Coverage remained the same at 100.0% when pulling ae8b888 on quaternion-maintenance into 7a3009f on master.