Release 0.2.0

1. Deprecated the old setup.py; now use pyproject.toml.
2. Added semantic typings for better documentation.
3. Added support for SDEs with time-dependent drift/dispersion, as well as for target function \phi. The time variable support is not implemented for tme.mean_and_cov as we don't have a consistent approximation for the covariance part for now. Note that it was not implemented for the sympy module either.
4. Migrated all the unittest to pytest.
5. Added an experimental feature for future work.