make task entries json serializable (#443)

* make sympy-based task entries json serializable

* remove datetime objs from time_intervals metadata

* make adv geometry json serializable

* make futoshiki metadata json serializable

* fixes

* futoshiki tweaks

* fix adv geometry

* deal with fractions in str representations

* fix

* restore start_time, end_time as str

reasoning_gym/algebra/intermediate_integration.py

@@ -242,7 +242,6 @@ Use same variable symbols as given in the question
                 "integrand": str(integrand),
                 "problem_type": problem_type,
                 "variable": str(x),
-                "expected_answer_expression": answer,
                 "difficulty": {
                     "problem_type_weights": self.config.problem_type_weights,
                 },

reasoning_gym/algebra/polynomial_multiplication.py

@@ -114,7 +114,7 @@ When performing calculations, please follow these guidelines:
                 "source_dataset": DATASET_NAME,
                 "source_index": idx,
                 "polynomial_expr": str(polynomial_expr),
-                "variables": list(product.free_symbols),
+                "variables": [str(x) for x in product.free_symbols],
                 "difficulty": {
                     "min_terms": self.config.min_terms,
                     "max_terms": self.config.max_terms,

reasoning_gym/algebra/simple_integration.py

@@ -88,7 +88,6 @@ When performing calculations, please follow these guidelines:
                 "source_index": idx,
                 "integrand": str(derivative),
                 "variable": str(symbol),
-                "expected_answer_expression": polynomial,
                 "num_terms": num_terms,
                 "difficulty": {
                     "terms": (self.config.min_terms, self.config.max_terms),