iteration process in the following attributes:
       *iparams*, *fparams*, *bparams* - dictionaries of integer,
       float, and boolean valued input parameters, respectively

       *ivars*, *fvars*, *bvars*, *tvars* - dictionaries of
       integer, float, boolean, and Tensor valued iteration
       variables, respectively.

       *A*, *B*, *iK* - input Tensor arguments.

       *E*, *X*, *S*, *R* - iteration Tensor variables.

    For instance:

       *ivars["istep"]* - the current iteration step *X* - the
       current approximation of eigenvectors *E* - the current
       approximation of eigenvalues *R* - the current residual
       *ivars["converged_count"]* - the current number of
       converged eigenpairs *tvars["rerr"]* - the current state of
       convergence criteria

    Note that when *tracker* stores Tensor objects from the LOBPCG
    instance, it must make copies of these.
