Derive basal contacts from geology map in thickness calculators

[lachlangrose]

Summary

• The thickness calculators (GUI widget and Processing Toolbox algorithm) previously required a basal-contacts line layer to already exist, forcing users through a separate "Extract Basal Contacts" step first.
• Basal contacts are now optional: when omitted, they're extracted automatically from the geology layer and stratigraphic column, reusing the existing extract_basal_contacts / map2loop ContactExtractor logic (no duplication).
• Fixes a related gap where the "AlongSection" calculator type skipped geology/basal-contacts validation entirely in the widget.

Test plan

• In QGIS, open the Thickness Calculator dialog, select a geology layer and a defined stratigraphic column, leave "Basal Contacts Layer" empty, and confirm thickness calculates successfully.
• Confirm the existing manual-basal-contacts-layer workflow still works unchanged.
• Run the Processing Toolbox "Thickness Calculator" algorithm with the Basal Contacts parameter left empty.