Improvements

• step_umap() has gained initial and target_weight arguments. (#213)

• Calling ?tidy.step_*() now sends you to the documentation for step_*() where the outcome is documented. (#216)

• Documentation for tidy methods for all steps has been improved to describe the return value more accurately. (#217)

• {keras} and {tensorflow} have been moved to Suggests instead of Imports. (#218)