Add summary statistics for variables

[danlooo]

Add statistics e.g. mean and quantiles as meta data to the axes. Needed for plotting e.g. color bar limits. Also do not need to reload all the data just to get the means.

[danlooo]

There is ZEP5 that provides summary statistics on a chunk level: https://zarr.dev/zeps/draft/ZEP0005.html
However, it is not merged yet zarr-developers/zarr-specs#205

[danlooo]

CF conventions have actual_range see https://cfconventions.org/cf-conventions/cf-conventions.html#missing-data to store min and max of each data variable.

[danlooo]

Remove manually setting limits of colorbar see #112
Adding CF meta data is still encouraged and possible.