Add missing sidre group methods

[rhornung67]

For consistency with other methods, we should add methods to the Group class:

• destroyGroupAndData()
• destroyGroupsAndData()
• destroyGroupSubtreeAndData()
• destroyGroupSubtreesAndData()

See #100 for additional context

[nselliott]

I have the following methods in progress. The AndData signifies that each will use the existing destroyViewAndData when destroying a View. This will destroy any Buffer that becomes attached to zero Views upon the destruction of its last View.

• void destroyGroupAndData(const std::string& path);

• void destroyGroupAndData(IndexType idx);
  These destroy a specific Group and its entire subtree.

• void destroyGroupsAndData();
  This destroys all Groups owned by this Group. The Views directly owned by this are not affected. There is an analogous destroyViewsAndData() already in the code.

• void destroyGroupSubtreeAndData();
  This destroys the entire subtree of this Group, both Views and Groups. this remains in existence as an empty Group.

At the top of this issue there is also the suggestion destroyGroupSubtreesAndData, but I am not sure what this would mean that is not redundant with what I have described here. Is there something more needed that I am overlooking?