Nested Map -> DataRow conversion

[Jolanrensen]

Fixes #1755

created new overloads for Map<*, *>.toDataRow() expanded with maxDepth and convertKeysToString to allow recursive map -> datarow conversion with key -> string handling

@JvmOverloads is added to avoid bytecode incompatibility between versions.

TODO: Iterable<Map<*, *>>.toDataFrame()

[Jolanrensen]

@AndreiKingsley @koperagen
One important thing: what to do about lists/iterables inside the map? Should we convert those too? A Iterable<Map<String, *>> can be seen and converted to a DataFrame after all, making Iterable<String, List<Map<String, *>>> a DataFrame with FrameColumns.

Plus, Map<String, Iterable<*>>'s can also be converted to a DataFrame. Should that be done?

(Also where have I seen this before... oh right JSON objects/arrays -> DataFrame XD)