Organize public-facing `pudl.transform` interface with selective imports

[zaneselvans]

• [ ] Use relative imports to bring general purpose classes into pudl.transform.* namespace:
  • [ ] AbstractTableTransformer
  • [ ] TransformParams
  • [ ] MultiColumnTransformParams
  • [ ] TableTransformParams (ambiguity here... one transform or all transforms?)
  • [ ] RenameColumns
  • [ ] StringNormalization
  • [ ] StringCategories
  • [ ] UnitConversion
  • [ ] ValidRange
  • [ ] UnitCorrections
  • [ ] InvalidRows
• [ ] import pudl.transform.params.ferc1.TRANSFORM_PARAMS as TRANSFORM_PARAMS_FERC1
• [ ] Import or relocate general purpose parameter definitions into pudl.transform.params.* namespace
  • [ ] unit conversions
  • [ ] valid ranges

Note: Do NOT import the individual transform functions, since they're meant to be used as methods.

Questions:

Should the function/parameters be split out into a separate module from the AbstractTableTransformer, since both of those are likely to grow? Seems like no... If we have people access them via pudl.transform.* namespace, then we can move them around later as needed without breaking a bunch of code -- we'll just need to change where they get imported from in __init__.py

Should the individual transform functions and multicolumn transform functions which are wrapped in methods be made _private(), to indicate that they are (primarily) meant to be accessed as methods in the TableTransformer classes?

[zaneselvans]

I created hierarchical imports in all the init.py files, but am putting off this bigger interface organization question until later after conversation with @cmgosnell