Implementation of mode selection strategies

[PeterGerds]

The modal truncation performed in HAMLS can be performed using many different approaches (see literature for different mode selection strategies):

• Implement a mode selection strategy where eigenvectors are selected based on a given or adaptive truncation bound. See, for example, Remark 8.2 in [Peter Gerds: "Solving an Elliptic PDE Eigenvalue Problem via Automated Multi-Level Substructuring and Hierarchical Matrices", Dissertation, RWTH-Aachen University].
• Implement a generic interface where the user can pass a function to the program that can specify the eigenvectors to be selected depending on the size of the subproblem, the problem type (subdomain/interface), underlying dimension and used polynomial degree of FEM-discretisation.