Handle JSON data type in both directions

[mkrebs81]

Situation

The Python library does not support the json data type yet.
It will soon be required in SGCP and dynamic tariff applications.

The first step should be the implementation of JSON values as strings, which could later be changed to actual JSON node objects.

Writing JSON data also needs an implementation which supports transformations using JMES mappings etc.

Tasks

• implement conversion where necessary
• implement write method with mappings / transformations

References

See the same issue in Java: SmartGridready/SGrJava#149

[mkrebs81]

Current implementation (0.5.x) can return JSON data as string or Python object (dict, list).

Transformations are not yet implemented in write direction.

Conversion between device type and data point type may be required.