DataConsistency and 2D plotting support

1. FEATURE: Explicitely marks the closure of the trace
2. BUGFIX: Introduces create_datum. create_dataset is updated and is now reserved for adding full datasets (shouldn't be edited later). If appending is needed first create_datum() and then follow it with repeated calls to append_datum().
3. BUGFIX: verify_type is stricter now. It goes beyond checking for root dtypes. It thoroughly checks for type, shapes, keys - even for nested structures. This implies that dict{datum} also needs to maintain its internal keys to support append leading to further consistency
4. FEATURE: Much like rendering of rgb datasets, now trace supports plotting of numerical datasets using a list(grp_keys)_ng and list(list(dataset_keys))_ng
5. Adds multiple tests to check for data consistency and plotting consistency