Support sparse, reduced versions of symplectic matrices

[apkille]

2n x 2n symplectic matrices can be computed as a reduced form known as the symplectic butterfly form, which can be written in block matrix form consisting two diagonal matrices (n x n) and two tridiagonal matrices (n x n), as discussed in this paper. Having support for a type SymplecticButterfly would be quite nice and yield more performant symplectic computations. Subsequent work could also be done regarding fast computations of symplectic eigenvalues.

[apkille]

The algorithm for computing the reduced butterfly form is given here. This paper also references some well known symplectic transformations, such as symplectic Householder, symplectic Givens, symplectic Gauss, which are neat and could also be custom types in this package.