Get tiff layers per group

### Prerequisites

- [x] I have written a descriptive issue title
- [x] I have verified that I am using the latest version of Magick.NET

### System Configuration
.net Framework 4.7.2

Magick.NET version: 7.9.0.2
Environment (Operating system, version and so on): Windows 10 1703 x64

### Question
Hi :)

Is it possible to get all tiff layers per group? In photoshop you can add a group/folder to get a better structure in the tiff.
For a software project this would be very useful.

example:
```

//some initialization
magickimagecollection.Read(file);
foreach(var layer in magickimagecollection) {
string group = layer.Group;
}
```
thanks for your help