Ipyexperiments

- add `ipyexperiments.utils.ipython` module with workaround for ipython memory leak on exception problem.
- add `ipyexperiments.utils.mem` module with memory debug helper utils.

- exp.data and exp.cl.data now both return namedtuples for the top-level entries, so it'll be easier to extend features in the future - documenting only the dictionary access, instead of raw tuples.
- cl: time delta is now printed in hh:mm:ss.msec

- integrate IPyGPULogger as a subsystem (renamed to CellLogger), so now there is no need to install and manage two modules that share a lot of similarities.
- fix a bug where ipyexperiment obj was getting deleted when a cell was rerun

- fix bug in memory Utilization calculation

- on GPU backend loading report the ID, Name and Total RAM of the selected GPU
- print_state now gives an easier to read report

Breaking changes

- made the module into proper subclasses, no more global function aliases. So now use directly the desired backend: `IPyExperimentsCPU`, `IPyExperimentsPytorch` as an experiments module. It should be trivial now to add other backends.
- and `get_stats` method has been replaced with `data` property method, which now returns one or more `IPyExperimentMemory` named tuple(s) depending on the used subclass.

- revert the attempt to account for modified variables - it doesn't work if some function during the experiment modified a variable introduced before the experiment - we surely must not delete it.