Implement Fetch Strategy support for non-correlated attributes

[beikov]

We already added the possibility to specify the fetch strategy on attributes with @Mapping(fetch = FetchStrategy.JOIN) but currently the value isn't considered. This is a follow up on #52 and is related to #108 as the implementations for the strategies came from the correlated subview area.

The first version of this will not do any grouping of operations but that should be ok since other strategies should only be rarely used.
Also update the metamodel documentation about flat views supporting non-joined collection mappings.