allows modifying list of aspect ratio

```obj-c
    PECropViewController *controller = [[PECropViewController alloc] init];
    controller.aspectNames = @[]; // empty
    [controller addAspectName:@"3 : 2" forRatio:3.0/2.0];
    [controller addAspectName:@"4 : 3" forRatio:4.0/3.0];
    [controller addAspectName:@"16 : 9" forRatio:16.0/9.0];
```