Improve the structure of DenovoSig

[Hu Jin]

We should follow more closely the design of sklearn estimators: https://scikit-learn.org/stable/developers/develop.html.

For example:

1. Remove parameter validation from __init__.
2. Add a reasonable set_params method.
3. Move the input X from __init__ to fit().

Ideally, we should also do this for other classes, e.g., MVNMF, NMF, etc.