Prevent DataMisfitController scheduler from modifying EKP obs_noise_cov

[costachris]

posdef_correct was modifying inputs in-place due to the for loop over diagonal elements in the symmetric matrix case. This resulted in the DMC scheduler directly modifying the EKP obs_noise_cov matrix in the first iteration. This PR:

1. Uses deepcopy to prevent in-place modification of the EKP obs_noise_cov and modify tests to detect this.
2. Exports posdef_correct for external use.

Note: examples/LearningRateSchedulers is using UniformScaling and remains unaffected.

[odunbar]

LGTM! good find

[costachris]

bors r+

[bors]

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