improve groovy magic

[scottdraves]

following #5398 we still need:

1. return complex results, not just making a string of everything. try evaluating new Plot() or println("stdout") and make those work the same as in a regular groovy notebook. ditto for errors. Essentially, it's a Comm proxy.
2. reuse the kernel. looks like a new one is being created for each cell:
   

and

3. do not start the kernel until a groovy cell is evaluated.
4. why are there two source files (beakerx/groovy_magic.py and beakerx/beakerx/groovy_magic.py). seems it should all go in one.

[michalgce]

@scottdraves Ad.2 is done. Kernel is initialised once