Reviewed support for working with groupings

 - Using lowercase for calculated properties on groupings
 - Using mean rather than average to capture the mean of a property on a group
 - Improved documentation of Grouping and Dimension
You're right, this is confusing!

What count() actually gives you is the number of occurrences of a given property value.  See [Grouping.count() docs](https://github.com/niksoper/insight/blob/feature/review-groupings/src/mda/Grouping.js#L748) for the detail.  So the 'total' property is the total number of occurrences of the property in the dataset.  For example:

```javascript
// example group object that has been counted
{
  "key": "Male",
  "value": {
    "count": 4,
    "height": {
      "160": 2,
      "169": 1,
      "172": 1,
      "total": 4
     }
  }
}
```

The difference between `value.count` and `value.height.total` in the above example is that `value.count` is the number of objects in the group whereas `value.height.total` is the number of objects in the group *with a height property*.

The problem is that we already have a 'count' property on the group value, so perhaps the total property should be renamed **'totalCount'**?