Integrate seaborn plotting capabilities

The seaborn package has very wide-ranging plotting capabilities that would greatly expand the capabilities of dantro.

Proposal

• Add seaborn as dependency
  • Should not be a big issue as it is very well maintained and widely used
• Make it easy to use seaborn plotting functions
• Implement a seaborn_grid function (name up for discussion) to allow using all seaborn plot kinds (analogue to the factet_grid plot) that create a figure themselves.

Tasks

• #227 (closed) implements a first version of the multiplot function (see complete discussion below): Plot multiple functions on a single axis, allowing for various plot functions including seaborn but also matplotlib.
• Figure out a consistent interface design
• Make figure-level plots available
• #238 Make theming and palettes availables