Better tests for estimators

[grfrederic]

it would be nice to organize our estimator tests, that is have a generic testing function:

test_estimator_on_task(estimator, task, n_samples, seed, abs_error, rel_error)

and then use it to build our tests:

1. a generic/easy group for all estimators
2. R and julia estimators can be optionally tested (with the same tests?)
3. longer/advanced tests for estimators we expect to perform well (esp KSG and neural estimators)