Design of "cell_lineages" folder placement

[ehuan2]

Currently, we don't love the way that "cell_lineages" and the data is being stored. We need a yaml file that points to the cell lineages. In #93 , I moved this to be under scTimeBench/shared/datasets/cell_lineages because I wanted it to be tightly coupled with default_datasets.yaml/optional_datasets.yaml. But there are a couple of issues:

1. The path in the .yaml is now inconsistent with the data path, as the cell_lineages refers to the relative path, whereas the ./data is relative to the root directory of where scTimeBench gets called.
2. Cell lineages and data paths should ideally be coupled.
3. I still like the idea of keeping track of cell lineages, as it's easier for users to see which cell lineages are being used.

To be fixed soon.