Js heat equation

Implementation of heat transfer using current DG method.
- class TransportDG implements a numerical method for solving general advection-diffusion equation and is templated by class Model, from which it inherits methods for computing the coefficients of the equation
- the EqData sub-class is first defined in Model and later inherited in TransportDG where further fields are added, that are related to the numerical method itself. Similar process applies to the definition of the input_type record.
- coefficient sigma_c is now calculated internally from diffusion tensor and cross section
- solution of linear algebra systems in transient problem starts from previous solution