Output trees in JSON format

[BauerLab]

Implement a readable output e.g. JSON for the trained trees. This will help evaluate the resulting interactions and visualise them.

[piotrszul]

Implemented: -omf option can be now used to define the format for model saved with -om Valid formats are:

• java (default) for binary java serialization format
• json for test JSON format

e.g.:

variant-spark importance .... -om 'model.json' -omf json

[rocreguant]

implemented