[Cleanup] Changing flow formulations from (oldField,deltaField) to (oldField,newField)

[francoishamon]

The flow solvers use formulations based on (oldField,deltaField), such as (pressure,deltaPressure). Is it too late at this point to think about changing these formulations from oldField, deltaField to oldField, newField? With this change, a lot of extra code could be eliminated.

Originally posted by @klevzoff in #1225 (comment)