What to save in checkpoints?

[landinjm]

There is some decisions we have to make when saving checkpoints. Currently we save the triangulation, all solution fields, the time, and the increment. If old solution fields are stored, those should also be saved.

We may want to include some more runtime parameters such as:

• Element degree
• Boundary conditions

Additionally, certain fields may not need to be saved. AUXILIARY and TIME_INDEPENDENT and determined at each step entirely by other fields.

@fractalsbyx @zachcroft @david-montiel-t thoughts?