odeint in TMB

Barebone templates for implementing ODE models in TMB. Mainly to avoid Type and double conflicts.

Steps:

1. Define the system using the example from the class harm_osc, the model must be able to initialize itself when taking the parameter vector as input.
2. Call the model with the class ODE<Type, model<Type> > and initialise it with
   1. initial conditions (e.g., DATA_VECTOR)
   2. parameters (e.g., PARAMETER_VECTOR)
   3. tmax: max time of the ODE (DATA_SCALAR)
   4. dt: time steps (DATA_SCALAR)

Data is stored simply as .track - a TMB (Eigen) vector, which can be easily reshaped, mapped to other purposed.

The system solutions and time steps can be retreived with out() method.

Compile

Add BH headers to the include path, e.g.,

TMB::compile("ode.cpp", paste0("-I", qsystem.file("include/", package = "BH")))

Harmonic Oscilator