CoreML C# Binding Generator uses wrong protection level

### Steps to Reproduce

1. Create a Xamarin.Mac application
2. Add a CoreML model e.g. Google Places 365
3. Use the model e.g. with Vision framework

### Expected Behavior

The application will compile.

### Actual Behavior

The application has a protection level compile error

### Environment

```
var modelRaw = new googlenet_places365();
var modelVision = VNCoreMLModel.FromMLModel(modelRaw.model, out vnErr);
```

### Build Logs

_ViewController.cs(32,57,32,62): error CS0122: 'googlenet_places365.model' is inaccessible due to its protection level_

### Fix

The CoreML binding generator needs to insert public before the model property

```
public readonly MLModel model;