Add support for quaternions to GaussianMixture and related

[xEnVrE]

Within this PR:

• added support for quaternions to GaussianMixture and derived classes (Gaussian and ParticleSet).
• updated test_Gaussian
• updated CHANGELOG.md

Details

The most important change is done inside the class GaussianMixture where a new argument use_quaternion is supported in the generic constructor. By default, the constructor assumes that use_quaternion = false.

The interpretation is the following:

• when use_quaternion == false and dim_circular != 0, then each circular component is treated as a component in the manifold S1, i.e. the circle. Since GaussianMixture, and derived, are mainly responsible for the storage, the mean circular components are stored as vectors in R^{dim_circular} and the associated covariance matrices as matrices in R^{dim_circular x dim_circular}. This is the default behavior that the library was already implementing.
• when use_quaternion == true and dim_circular != 0, then each circular component is treated as a component in the manifold S3 represented using a quaternion. Storage-wise, the mean associated with each quaternion is stored as a vector in R^4(i.e. real and imaginary part of the quaternion) and the associated covariance matrix as a matrix in R^{3 x 3}. Indeed, the uncertainty associated with the quaternion is best represented in the tangent space, i.e. rotation vectors, that can be treated as R^{3}.

[claudiofantacci]

Just some minor changes, then LGTM 🚀

[claudiofantacci]

@xEnVrE, can you please resolve the conflicts? 😄

[xEnVrE]

@xEnVrE, can you please resolve the conflicts? 😄

Yep!