Delta Summary Case: Avoid computing _DIFF vectors

[magnesj]

When history data is present for summary data imported from a data source, a difference summary vector is automatically created by resinsight. Example FOPT and FOPTH are available, FOPT_DIFF is displayed as a vector to select, and is computed as FOPT_DIFF = FOPT - FOPTH for each time step.

This computation is considered to be confusing in a Delta Summary Case and Delta Ensemble.