[belisoful]

Follow-up to #8.

• [668] SQLMap groupBy incorrectly handles left join results where right part is empty (null) #4: a nested result map with its own groupBy grouped across parents: the same child value under two parents shared one tree key, so the second parent lost the element and the first parent's element was replaced. Nested keys are now prefixed with the parent key. Adds unit tests for partial-null nested rows, two-level nesting, column-index and no-column maps, and a GroupByTest LEFT JOIN case that runs against every database.
• CacheModel attribute superflous/not used for <update> and <insert> statements #5: documents the cacheModel attribute in the insert, update, delete and cacheModel reference pages. Adds unit tests for a cache model declared in another mapping file, a statement both listed in flushOnExecute and carrying the attribute, and a CacheTest case that runs against every database.
• "Prado\Data\SqlMap\DataMapper\TPropertyAccess::get" do not work when object instanceof \ArrayAccess #6: corrects the @throws tags of TPropertyAccess::get()/set() and documents path resolution. Adds tests with a custom ArrayAccess class that proves offsetExists()/offsetGet() are used, plus null values, nested misses and set() on an ArrayAccess root.