Fix EKF measurement shape mismatch during manual step-by-step updates

[leorouxx]

Description

Summary

This pull request resolves issue #135, addressing a measurement shape mismatch that occurs during manual step-by-step updates in the Extended Kalman Filter (EKF) implementation when an EKF object is initialized empty, and magnetometer data is provided in the update function.

Changes Made

update() Method: Introduced a local use_mag = mag is not None boolean flag to guarantee consistent evaluation of magnetometer presence throughout the update step, replacing redundant inline conditional checks.

Expected Measurement Function (h): Updated the function signature and header to accept the use_mag flag, ensuring the correct measurement vector shape is constructed.

Linearized Measurement Matrix (dhdq): Added the use_mag flag to the header to align with the revised measurement formulation.

Documentation: Updated the docstrings for both h and dhdq to accurately document the new parameter usage and behavior.

Tests: Added a unit test in TestEKF class named test_step_by_step_with_mag. This test simulates the behavior pointed out in #135 and now passes.

Explanation

I choose a solution with a boolean flag, that may be considered inelegant at first glance, but h and dhdq are stateless functions that are called only inside update.