Fix broken tests by including required assets in package_data

[carlthome]

Working on packaging madmom in nixpkgs (WIP here) and noticed that specific tests failed when testing the installed (non-editable) package by:

MCVE

git clone https://github.com/CPJKU/madmom.git
cd madmom

git submodule init
git submodule update

python -m venv .venv
source .venv/bin/activate

pip install .
pip install pytest

rm -r madmom

pytest

Output

====================================================== short test summary info ======================================================
FAILED tests/test_bin.py::TestTCNBeatTrackerProgram::test_binary - IndexError: list index out of range
FAILED tests/test_bin.py::TestTCNBeatTrackerProgram::test_run - IndexError: list index out of range
FAILED tests/test_bin.py::TestTCNTempoDetectorProgram::test_binary - IndexError: list index out of range
FAILED tests/test_bin.py::TestTCNTempoDetectorProgram::test_run - IndexError: list index out of range
FAILED tests/test_features_beats.py::TestTCNBeatProcessorClass::test_process_tcn - IndexError: list index out of range
====================================== 5 failed, 806 passed, 188 warnings in 60.53s (0:01:00) =======================================

Turns out these tests depend on static assets (2019 models) that are currently not included in package_data. This PR fixes that such that the installed package passes its tests.

[carlthome]

Already fixed in #531 - closing!