Remove calculation of statistical surfaces in plugins

**Is your feature request related to a problem? Please describe.**
Statistical calculation should be done as a preprocess due to performance issues in existing plugins.

**Describe the solution you'd like**
Standard storage location for statistical surfaces for FMU scratch ensembles
Loose link to storage in plugins such that it is easy to switch to future storage solutions

Mainly relevant for plugins:
- SurfaceViewerFMU
- WellCrossSectionFMU

**One suggested location of statistical surfaces**
*[ensemble_set]/share/results/maps/[ensemble]/[statistics]/[name]--[property]--[date].gri* ,
where `statistics` is e.g. `mean`, `p10`, `min`.

[Example from test data](https://github.com/equinor/webviz-subsurface-testdata/tree/master/reek_history_match/share/results/maps)

Should we also allow for different "sets" of statistical surfaces, e.g. per senscase?
`[ensemble_set] / share / results / maps / [ensemble] / [set] / [statistics] / [name]--[property]--[date].gri` ,
where `set` is e.g. `senscase` name ?