Easy insertion of equations into the model at specific position

[luyckxn]

E.g. add equation CP=A_CENTRAL/VC into the ODE code record.

[luyckxn]

From now on, position argument 'pos' available in method 'add' also allows code records elements.

For instance, to add an equation CP=A_CENTRAL/VC into the ODE code record, you can proceed as follows:

model <- model %>% add(Equation("CP", "A_CENTRAL/VC"), pos=Position(OdeRecord(), after=<logical>))

after=TRUE (default) -> equation is appended at the end of the ODE record.
after=FALSE -> equation is appended at the beginning of the ODE record.

Please note that Position() also accepts any model statement, as shown below.

model <- model %>% add(Equation("CP", "A_CENTRAL/VC"), pos=Position(Ode("A_OUTPUT"), after=<logical>))

In this case, the equation is added before or after the ODE 'OUTPUT' depending on 'after'.