Solve issues with handling of group items and functions.

[splatch]

This should solve #27.

[cdjackson]

Hey @splatch I'm wondering if this is still relevant or if this was included in the recent OH updates as I seem to recall a lot of changes in OH around the group functions (IIRC, which I might not :) ).

[splatch]

Hey @splatch I'm wondering if this is still relevant or if this was included in the recent OH updates as I seem to recall a lot of changes in OH around the group functions (IIRC, which I might not :) ).

To be fair I didn't track OH work in this area hence I can't tell you if its needed. From quick look at most recent sources of OH I think these changes are still relevant. OH did not solve issue (they didn't have) with access to GroupFunctionHelper from ItemDTOMapper since they have everything in single module. For us cause of split of items, things and other core components visibility of function helper is issue. The intermediate workaround supplied by this PR was to hide function helper behind item builder which is available within mapper calls.