Implement Mixed subtable methods

[bmunkholm]

For C++ highlevel:
A Mixed column with cells being subtables, poses some challenge. So for this we could make the following API:

// Create a subtable of the type T at the specific cell
createSubtable()

// Get the subtable of type T at the specific cell
getSubtable() (return NULL if not compatible with type T)

// Return true if the cell is of type T
bool isSubtable() (helper for above)

// Schema not defined at compile time, dynamic during runtime:
"DynamicSchemaTable" getSubtable()
+ methods for looking at the spec and updating it

[kspangsege]

Some of this has been done (maybe all of it).

Note: Names are not supposed to be camelCased in our C++ API.