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

Start with the MagickImageCollection.Read call and the proposed layer.Group access shown in the issue. Determine whether grouped TIFF layers can be exposed through this API and define the expected group structure before implementation.

Scritto dal modello di indicizzazione a partire dal testo della issue.