Reject blank GUI erosion rate

[sergioald]

Summary

Rejects a blank value in the GUI bank-erodibility / erosion-rate field instead of silently restoring the historical 1.0e-8 default.

Why

PR #41 used _safe_tk_string(..., '1e-8') while building the GUI run configuration. Clearing the required erosion-rate field therefore caused the GUI to continue with 1.0e-8 without warning.

That is unsafe for sensitivity studies because the submitted run can differ from the value the user intended.

Changes

• adds a required-float GUI parser with a clear error for blank values;
• uses it for the erosion-rate field;
• retains the historical default only when creating a new GUI or loading an older configuration that lacks the field;
• adds a regression test covering blank rejection and valid scientific notation.

Validation

python -m py_compile gui_ldsfl.py tests/test_erosion_rate_control.py
python -m ruff check gui_ldsfl.py tests/test_erosion_rate_control.py
python -m pytest tests/test_erosion_rate_control.py
python -m pytest