Validate assignment onto base classes

Currently, some fields of `Learner`, `Task`, `Measure`, `Resampling` are simple fields and no input validation is performed. 

E.g. the following works

```r
library(mlr3)
l = lrn("classif.rpart")
l$timeout = "invalid_value"
```


We should add input validation for:

* [ ] Learner
* [ ] Task
* [ ] Resampling
* [ ] Measure
* [ ] ResampleResult
* [ ] BenchmarkResult