The EquiSolnAlgo class is an abstract base class. Its
purpose is to define the interface common among all subtypes. An
EquiSolnAlgo object defines the sequence of operations
performed by the the Integrator and the LinearSOE objects in
solving the equilibrium equation R(U) = 0 given the current state of
the domain at each time step in a direct integration analysis or load
increment in a static analysis.