Add a flag to choose what fields are saved to vtu

[landinjm]

For grain growth, it's much better to save the post-processed fields to vtu rather than each individual grain.

Adding some flag in userInputParamters or variableAttributes.

@zachcroft Feel free to add more information.

[zachcroft]

The motivation for this is to reduce the VTU file size. Running large 3D simulations with dozens of order parameters can result in VTU files several GB in size, and saving these outputs as well as opening them in visualization software is time consuming. For the case of grain growth, it might be a good idea to just save the sum of order parameters squared, which gives the locations of the grain boundaries and is typically what's plotted during analysis. The file size should be much smaller, and other post processed fields such as the order parameter ID map or feature ID map could be useful too.