Find ways of visualizing Sole types & type trees.

We should find a way to visually conveying the most important Sole concepts in a mind map.

Here's code for plotting some important Sole type trees:
```
using Sole
using GraphRecipes, Plots

# SoleLogics.jl
plot(Sole.Formula, method=:tree, fontsize=10, nodeshape=:rect)
plot(Sole.AbstractAlphabet, method=:tree, fontsize=10, nodeshape=:rect)
plot(Sole.AbstractInterpretation, method=:tree, fontsize=10, nodeshape=:rect)
plot(Sole.AbstractInterpretationSet, method=:tree, fontsize=10, nodeshape=:rect)

# SoleData.jl
plot(Sole.AbstractFeature, method=:tree, fontsize=10, nodeshape=:rect)
plot(Sole.AbstractCondition, method=:tree, fontsize=10, nodeshape=:rect)
plot(Sole.AbstractLogiset, method=:tree, fontsize=10, nodeshape=:rect)
plot(Sole.MultiLogiset, method=:tree, fontsize=10, nodeshape=:rect)

# SoleModels.jl
plot(Sole.AbstractModel, method=:tree, fontsize=10, nodeshape=:rect)
# Interface: isopen, outcometype, outputtype
```