工具方法使用方法：
///模型存储 - 数据持久化保存的key
+ (void)mn_saveModel:(id)model key:(NSString *)key;
///读取 - 获取数据持久化保存的Model - 要传入model的Class - 内部进行yyModel转模型
+ (id) mn_getSaveModelWithkey:(NSString *)key modelClass:(Class)modelClass;
///模型数组存储 - 获取数据持久化保存的Model - 要传入model的Class - 内部进行yyModel转模型
+ (id) mn_getSaveModelWithkey:(NSString *)key modelClass:(Class)modelClass;
///获取保存的模型数组
+ (id) mn_getSaveModelArrayWithkey:(NSString *)key modelClass:(Class)modelClass;
实际操作：
- 存储
[MNCacheClass mn_saveModel:_studentModel key:@"studentModel"];
- 读取
StudentModel *get = [MNCacheClass mn_getSaveModelWithkey:@"studentModel" modelClass:[StudentModel class]];