Make TechnologyManager technology accessors privately mutable

Add InventionManager as a friend of TechnologyManager
Remove const_cast from `InventionManager::generate_invention_links`

Author: Spartan322

src/openvic-simulation/research/Invention.cpp

@@ -146,11 +146,11 @@ bool InventionManager::generate_invention_links(TechnologyManager& tech_manager)
 
 	for (Invention& invention : inventions.get_items()) {
 		for (const memory::string& tech_id : invention.raw_associated_tech_identifiers) {
-			Technology const* tech = tech_manager.get_technology_by_identifier(tech_id);
-			if (tech) {
-				auto* mutable_tech = const_cast<Technology*>(tech);
-				mutable_tech->add_invention(&invention);
+			Technology* tech = tech_manager.get_technology_by_identifier(tech_id);
+			if (tech == nullptr) {
+				continue;
 			}
+			tech->add_invention(&invention);
 		}
 
 		invention.raw_associated_tech_identifiers.clear();

src/openvic-simulation/research/Technology.hpp

@@ -96,11 +96,15 @@ namespace OpenVic {
 	};
 
 	struct TechnologyManager {
+		friend struct InventionManager;
+
 		IdentifierRegistry<TechnologyFolder> IDENTIFIER_REGISTRY(technology_folder);
 		IdentifierRegistry<TechnologyArea> IDENTIFIER_REGISTRY(technology_area);
 		IdentifierRegistry<Technology> IDENTIFIER_REGISTRY_CUSTOM_PLURAL(technology, technologies);
 		IdentifierRegistry<TechnologySchool> IDENTIFIER_REGISTRY(technology_school);
 
+		IDENTIFIER_REGISTRY_NON_CONST_ACCESSORS_CUSTOM_PLURAL(technology, technologies);
+
 	public:
 		bool add_technology_folder(std::string_view identifier);
 
@@ -132,4 +136,4 @@ namespace OpenVic {
 
 		bool parse_scripts(DefinitionManager const& definition_manager);
 	};
-}
+}