DOF_Group is a base class. An object of type DOF_Group
represents an unconstrained node of the domain in the model. Each node
in the domain is associated with one DOF_Group. DOF_Groups are
called upon in the analysis to provide their contributions of
unbalanced load to the system of equations. Subclasses are used by the
constraint handler to to introduce new dofs into the analysis.
The DOF_Group is responsible for providing operations to set and
access the mapping betwwen equation numbers and DOFs, allowing the
Integrator to from the tangent (if nodal masses in transient problem)
and unbalanced load information, and for setting and obtaining the
nodal trial response quantities.
DOF_Group(int tag,