Histogram aggregation produces one plot per ensemble member

[daflack]

Describe the bug

When running histogram aggregations the finest grading you can get for a total histogram across all times is per ensemble member rather than for the full ensemble.

How to reproduce

Steps to reproduce the behaviour:

1. Select a histogram plot, aggregation over all.
2. See that a histogram is produced for each member, not the overall enesmble.

Expected behaviour

A single histogram using all ensemble members is created.