表达式函数，增加DateTime/TimeSpan，并且开始测试与整理

FreeSql.Tests/MySql/Expression/MathTest.cs

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+using FreeSql.DataAnnotations;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using Xunit;
+
+namespace FreeSql.Tests.MySql.Expression {
+	public class MathTest {
+
+		ISelect<Topic> select => g.mysql.Select<Topic>();
+
+		[Table(Name = "tb_topic")]
+		class Topic {
+			[Column(IsIdentity = true, IsPrimary = true)]
+			public int Id { get; set; }
+			public int Clicks { get; set; }
+			public int TestTypeInfoGuid { get; set; }
+			public TestTypeInfo Type { get; set; }
+			public string Title { get; set; }
+			public DateTime CreateTime { get; set; }
+		}
+		class TestTypeInfo {
+			public int Guid { get; set; }
+			public int ParentId { get; set; }
+			public TestTypeParentInfo Parent { get; set; }
+			public string Name { get; set; }
+		}
+		class TestTypeParentInfo {
+			public int Id { get; set; }
+			public string Name { get; set; }
+
+			public List<TestTypeInfo> Types { get; set; }
+		}
+
+		[Fact]
+		public void PI() {
+			var data = new List<object>();
+			data.Add(select.Where(a => Math.PI + a.Clicks > 0).ToSql());
+		}
+		[Fact]
+		public void Abs() {
+		}
+		[Fact]
+		public void Sign() {
+		}
+		[Fact]
+		public void Floor() {
+		}
+		[Fact]
+		public void Ceiling() {
+		}
+		[Fact]
+		public void Round() {
+		}
+		[Fact]
+		public void Exp() {
+		}
+		[Fact]
+		public void Log() {
+		}
+		[Fact]
+		public void Log10() {
+		}
+		[Fact]
+		public void Pow() {
+		}
+		[Fact]
+		public void Sqrt() {
+		}
+		[Fact]
+		public void Cos() {
+		}
+		[Fact]
+		public void Sin() {
+		}
+		[Fact]
+		public void Tan() {
+		}
+		[Fact]
+		public void Acos() {
+		}
+		[Fact]
+		public void Asin() {
+		}
+		[Fact]
+		public void Atan() {
+		}
+		[Fact]
+		public void Atan2() {
+		}
+		[Fact]
+		public void Truncate() {
+		}
+	}
+}