Separate function to compute algebraic variables

Currently algebraic variables are computed by the same function "dynamic" than the derivatives. However the values of the algebraic variables are usually thrown away when the ODE is solved. When the algebraic variables are computed the, the derivatives are thrown away.

This simplistic approach wastes computing power, therefore the algebraic variables should be computed by a separate function. This is interesting when PDEs are introduced to the language.

To implement this, the construction of a data flow graph, and the removal of dead code, must be implemented.

https://sourceforge.net/apps/trac/freeode/ticket/32