Visualise std of FIELDs (and SURFACES?)

[oyvindeide]

After https://github.com/equinor/ert/commit/501709ce4a39f92ef0dc9973cbda81b11c625176 we now have an endpoint for calculating std for parameters, would be good to visualize this in the plotter. Something like a per layer std for FIELDS for example. First pass could for example be making images on the server and passing those through the API if that is easier.