/*! \defgroup ExternalODESolverInterface Interface to external ODE Solver Libraries

The purpose of these classes and functions is to provide a general interface to
external ODE solver libraries. The interface is designed s.t. it can be used
without having to know which particular external library will be used to solve
the ODE. Furthermore all user-side (or OGS6-side) computations are done using
Eigen types. These types are convenient to use and provide bounds checking, even
at compile-time.

The inheritance and cooperation between the classes involved when solving an ODE
is depicted in the image below. The user side is at the top, the external
library side at the bottom. Sundials' CVode solver has been chosen as an example
since it is the first external solver we use.

The path on the left side is essentially the way data coming from the user takes
towards the ODE solver backend. On that way Eigen types are stripped to raw
pointers (<tt>double*</tt>).

The path on the right side is the way data coming from the ODE solver backend
takes towards the functions provided by the user. On that way raw pointers are
wrapped into some <tt>Eigen::Map<></tt>s.

\image html  external-ode-solver-concept.png "Inheritance/Cooperation between different classes, and data flow when computing y_dot = f(t,y)."
\image latex external-ode-solver-concept.pdf "Inheritance/Cooperation between different classes, and data flow when computing y_dot = f(t,y)."

*/