rm pressure, h_tot from cache and begin refactoring velocity cache

[akshaysridhar]

TODO: lazy expressions are materialized in several places where Fields.field_values and Fields.level are accessed.

[tapios]

Thanks, @akshaysridhar. This is a step in the right direction. However, it overlaps with what @costachris is working on: main...cc/remove_precomputed_new

Please coordinate this with each other. It would be good to get Costa's changes in and then build on top of that.