More options for in-memory objects

[david-cortes]

This PR introduces several changes in order to work better with in-memory data:

• Makes the functions accept sparse matrix objects as inputs.
• Leaves the option to store the fitted matrices in the model object in memory instead of creating a file, and access them directly for easier retrieval.
• Changes the default options for some arguments to avoid accidentally creating files where it's not intended.

[yixuan]

This looks great. Thank you @david-cortes! I'll find a time to take a look and merge it.

[yixuan]

Hi David, I have merged your PR with a few changes:

1. The matrix data source is now handled by a new function data_matrix(), to keep the interface clean and consistent.
2. The model can export P and Q to files even if the model matrices are stored in memory. (In the original PR it will stop.)
3. I finally choose not to explicitly output a matrix in predict(), since it makes the interface inconsistent. Instead, the user can easily construct a predicted sparse matrix by calling Matrix::sparseMatrix. (But maybe we can write a new output format such as output_matrix()?)

Again, I apologize for the late reply and thank you for the great work! I have added you as an author in the upcoming new version.