Improve summary module

[pnorton-usgs]

Improve summary module

Refactor and harden the pyPRMS/summary/ module (OutputCSV, OutputVariable, OutputVariables) with code quality improvements and initial test coverage.

Changes

Code quality

• Modernize type hints to Python 3.10+ syntax (X | None, dict, list)
• Remove redundant (object) inheritance from all summary classes
• Add missing docstrings and return type annotations
• Add __all__ export lists to each summary submodule
• Add __repr__ methods to all summary classes

Error handling

• Add file existence checks in OutputCSV and OutputVariable constructors (raise FileNotFoundError with descriptive message)
• Use descriptive KeyError messages in OutputVariables.get() and write_netcdf()

Refactoring

• Decompose OutputVariable.to_xarray() into focused helper methods (_resolve_dim_name, _build_data_array, _set_time_encoding, _set_variable_attrs) with module-level constants
• Replace match statement with if for Python 3.10 compatibility
• Move is_global metadata mutation out of cached_property into a dedicated _set_global_flags() method

Tests

• Add functional tests for OutputCSV covering streamflow-only and combined basin+streamflow CSV formats
• Add tests for OutputVariables reading and variable access
• Add tests for FileNotFoundError handling in both OutputCSV and OutputVariable

Target branch

development

Testing

All 301 tests pass.