Reading data: Add auxiliary pressure variable as dependency in task graph for other variables

Fixes #353 (closed)

In the current implementation, reading 3-D model fields with auxiliary pressure will often read the 3-D pressure field multiple times. This is because the 3-D pressure field gets read 'directly' from any other 3-D variable, instead of having the pressure field as dependency in the task graph. Because of this "harcoded" read call, multiple variables might read the pressure field concurrently.

This MR adds the pressure field as parent in the task graph of auxiliary pressure variables. Using the task graph, the pressure field is only read once and reused by all variables.