v0.6.1

Internal cleanup pass and a few small breaking removals. For older Python or
older import paths, pin a previous livelossplot version.

Removed

• The livelossplot.keras / livelossplot.tf_keras / livelossplot.poutyne /
  livelossplot.pytorch_ignite import shortcuts (deprecated since 0.5.0).
  Use livelossplot.inputs.* or from livelossplot import PlotLossesKeras etc.
• PlotLosses.draw() (deprecated since 0.5.0). Use .send().

Changed

• ExtremaPrinter parameter massage_template renamed to message_template.
• Plot2d parameter valiation_data renamed to validation_data.

Fixed

• Mutable default arguments in PlotLosses(outputs=...) and MatplotlibPlot(extra_plots=...).
• MainLogger.groups = None now correctly resets to {}.
• ipython declared as a runtime dependency.