Clean up and refactor Cbh.py

[pnorton-usgs]

Clean up and refactor Cbh.py

Summary

Modernizes type hints, refactors the long __init__ into focused methods, improves error handling, and cleans up code style in Cbh.py.

Changes

Modernize type hints

• Add from __future__ import annotations
• Replace typing.Dict, List, Optional, Union with built-in generics and X | Y syntax
• Remove unused typing imports
• Fix verbose and engine parameter types: Optional → plain types

Refactor __init__

• Extract _normalize_paths() for path handling logic
• Extract _read_netcdf(), _read_zarr(), _read_ascii() from __init__
• __init__ is now a clear orchestrator (~30 lines vs ~100)
• Add default case to engine match that raises ValueError for unknown engines (previously left ds unbound)
• _read_zarr() now raises ValueError instead of printing errors and leaving ds unbound

Use context manager in write_ascii

• Replace open()/close() with with open() context manager
• Ensures file is properly closed even if an exception occurs

Add __repr__

• Returns Cbh(src_path=[...], variables=N) for debugging

Move module-level constants to class constants

• var_crosswalk → _VAR_CROSSWALK
• temp_units → _TEMP_UNITS
• precip_units → _PRECIP_UNITS
• NA_VALS_DEFAULT → _NA_VALS_DEFAULT
• DATA_SEP → _DATA_SEP
• Better encapsulation: only used by the Cbh class

Code cleanup

• Remove commented-out dead code
• Replace 6 repeated pd.to_numeric() calls with a loop

Testing

All 290 tests pass.