4.0 target features

[FlipperPA]

• Remove support for SQL Alchemy < 2
• Deprecate SAS inspired features for Pythonic equivalents
• Convert from pandas to polars for better memory management
• Give user access to the underlying SQL Alchemy connection
• Split README into "developer stuff" and "user stuff"
  • There are a lot of undocumented options covered in code comments which should be in a "user" README

[FlipperPA]

• Support both polars and pandas - with a preference for polars

Something like import as dfp (Data Frame Package):

POLARS = True
try:
    import polars as dfp
except ImportError:
    POLARS = False
    import pandas as dfp