Performance: Investigate performance for grid statistics using grid calculations

[magnesj]

Grid statistics on grid calculations can consume a lot of CPU and memory if the grid ensembles are large. Investigate how the performance is for large ensembles. Things to test:

• filtering on a small selection of time steps
• how recursive grid calculations work (use a grid calculation that depends on other grid calculations)
• memory usage when Clear Grid Calculation Memory is toggled on

[magnesj]

Tested on a representative grid ensemble in Equinor, and the performance was reported to be sufficient for now.