Add a method for calculation of simulation parameters in 0th timestep

[landinjm]

Currently, there is no way to calculate / update parameters in customPDE with simulation data in the first timestep or others. An example would be something like this:

for (unsigned int i = 0; i < dim; i++)
  {
    element_volume =  element_volume * userInputs.domain_size[i] / std::pow(2, userInputs.refine_factor);
  }

Having something like this would be nice to do some calculation of parameters without having to do that in every cell.