New interpretation/Visualization API

[lorenzoh]

Implements the ideas in #154.

To-Dos:

• abstract types and interfaces
• backends
  • Text
  • Makie
• High-level functions for use with
  • LearningMethods
  • Learners
• default implementation for WrapperBlocks
• Bounded block
• new text-based display for learning rate finder
• lazy-load Makie.jl with Requires.jl. This reduces using time from 22 secs. to 17 on my machine (10 of which seem to come from Flux.jl)
• tests
• update documentation
  • use show instead of plot everyhwere
  • update "How to visualize"
• update CHANGELOG

[lorenzoh]

Depends on ronisbr/PrettyTables.jl#143

[lorenzoh]

Example of high-level visualization derived from a learning method. Also, custom table printing formats supported in ShowText.