Added axes serialization to json for export

[k-dominik]

Thank you for having a look @btbest

regarding your question on missing resolution and description - these are optional (default parameters for AxisInfo construction are in place).

test script

```Python import vigra atjson = """ { "axes": [ { "key": "y", "typeFlags": 2 } ] } """ print(vigra.AxisTags.fromJSON(atjson)["y"].resolution) ```

So yes, the current code only adds the axis info required for ilastik to interpret it correctly.

[k-dominik]

thx @tischi!!!