task should allow for simple user defined characteristics

[berndbischl]

assume you run a simulation study.
then often you create tasks with different properties.

either we can allow to add these properties to a benchmark_grid, and preserve them in the result.
(currently this results in a weird error message which could also be improved)
a better option would be to allow something like this

task$characteristics = list(n = 5, p = 7), or task$settings
then we can easily later "unfold" this into the table of the BMR