first step towards weekly planning

planner/src/rules.rs

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+//! Rules used by the `planner`
+//! A rule is a constraint on valid solutions, but also provides insights
+//! and eventually inferences to optimize the solving process.
+
+// Nutritional values
+// - Per day : according to user profile (man: 2000kcal, woman: 1800kcal)
+// - Per meal : some meals should have higher nutrional values than others
+
+// Ingredients
+// - Per week : should use most of a limited set of ingredients (excluding
+// condiments, ...)
+// - To consume : must use a small set of ingredients (leftovers)
+//
+
+// Price
+// - Per week : should restrict ingredients cost to a given amount