Extend DeepLIIF for SDG

[wendywangwwt]

The main work in this PR includes

1. adapting SDG as part of the DeepLIIF model class, deprecating the existing SDG model class
2. updating the documentation

New DeepLIIF Model Class

The new deepliif model class now supports multiple input modalities instead of 1 and translation-only objective, both of which are the key features of the SDG (Synthetic Data Generation) model.

Key files impacted:

• deepliif/models/DeepLIIF_model.py: seg_gen=True condition is made optional, input fetch is able to handle an array of multiple input patches
• deepliif/data/aligned_dataset.py: multiple input patches for DeepLIIF model class when necessary
• deepliif/models/__init__.py: inference compatibility

Documentation Updates

The main readme file has been updated to reflect the latest status of the code base. In addition, a new file "Model Types.md" is added that documents details about the model classes implemented so far.

Note that

1. the embedded link in the main readme file that points to the model types readme will need to be updated
2. the files for the doc site needs to be re-generated

Tests passed for DeepLIIF model class. Test log can be accessed here.