nano: Data Visualisation and Model Selection

# Things to do:
# - Create functions for plots - see example vignette for plots to be produced 
# - Clean data.table output from single_pdp function - need to review if any errors in functions
# - Implement function for plotting tree diagram from model. Look into other visualisation packages
#   to plot the tree diagram - interactive features.
#   https://www.h2o.ai/blog/finally-you-can-plot-h2o-decision-trees-in-r/
# - See what other plots can be produced for model explainability e.g. shapl
#   http://docs.h2o.ai/h2o/latest-stable/h2o-docs/explain.html
# - Implement target encoding for nano_grid and nano_automl - NEED TO REVIEW 
#   http://docs.h2o.ai/h2o/latest-stable/h2o-docs/data-science/target-encoding.html
# - Look through H2O for features to implement and look through blogs/tutorials
#   https://www.h2o.ai/blog/
# - 3D pdp - will need to use another package instead of H2O to calculate the values --> partial function in pdp pacakge
#   https://bgreenwell.github.io/pdp/articles/pdp.html, https://www.rdocumentation.org/packages/pdp/versions/0.7.0/topics/partial