[SUGGESTION] Rename Conserved

[jpcoles]

Throughout the code a "conserved" field is used to mean variables (or arrays) that preserve their value between iterations. However, a conserved field in physics has a very specific meaning (e.g., a conserved vector field is a vector field that is the gradient of a scalar potential function). To avoid confusion about the nature of these variables another name should be used. For example, preserved or persistent.