Stable orthogonalization

Description

In case the call to Cholesky fails, we now switch to Gram-Schmidt orthogonalization (for this call only). This avoid crashes of the code and makes it more stable.

News snippet

Making the Cholesky orthogonalization more stable.

Checklist

• I have checked that my code follows the Octopus coding standards
• I have added tests for all the new features added in this request.