Clean up imports

[grfrederic]

We've made a lot of changes when moving to our new tasks/benchmark API. It would be nice to rethink which tasks/estimators etc should be exported by default. For example:

1. when importing bmi.benchmark, should functions for creating tasks (which live in bmi.benchmark.tasks) be reexported there and users create the tasks themselves, or should the tasks from the benchmark list be reexported under convenient names?
2. should external estimators be exported separately, or included in bmi.estimators? we could go with the latter and raise warnings when someone tries to initialize an estimator and R/julia/some necessary packages is not installed.