Adding loader for MDB-stem-synth dataset

[dsuedholt]

Description

Please include the following information at the top level docstring for the dataset's module mydataset.py:

• [x] Describe annotations included in the dataset
• [x] Indicate the size of the datasets (e.g. number files and duration, hours)
• [x] Mention the origin of the dataset (e.g. creator, institution)
• [x] Describe the type of music included in the dataset
• [x] Indicate any relevant papers related to the dataset
• [x] Include a description about how the data can be accessed and the license it uses (if applicable)

Dataset loaders checklist:

• [x] Create a script in scripts/, e.g. make_my_dataset_index.py, which generates an index file.
• [x] Run the script on the canonical version of the dataset and save the index in mirdata/indexes/ e.g. my_dataset_index.json.
• [x] Create a module in mirdata, e.g. mirdata/my_dataset.py
• [x] Create tests for your loader in tests/datasets/, e.g. test_my_dataset.py
• [x] Add your module to docs/source/mirdata.rst and docs/source/table.rst
• [x] Run black, flake8 and mypy (see Running your tests locally).
• [x] Run tests/test_full_dataset.py on your dataset.
• [x] Check that codecov coverage does not decrease.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 97.06%. Comparing base (9f797d2) to head (6f8ac77). Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #639 +/- ## ========================================== + Coverage 97.05% 97.06% +0.01% ========================================== Files 66 67 +1 Lines 7498 7537 +39 ========================================== + Hits 7277 7316 +39 Misses 221 221 ```

[dsuedholt]

Thanks for the friendly and helpful review @genisplaja! Let me know if there's anything else you need for the release.