unify importing in codebase

[Lioscro]

Should be a simple fix, but just posting as an issue so that we get to it some time.
We should unify local importing to use relative imports (i.e. from .data import CassiopeiaTree) instead of specifying the full package name (i.e. from cassiopeia.data import CassiopeiaTree).
As of right now, most submodules use the absolute importing scheme.