support multiple datasets in `arx`

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.05 :tada:

Comparison is base (8991b63) 86.70% compared to head (6023abc) 86.75%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #127 +/- ## ========================================== + Coverage 86.70% 86.75% +0.05% ========================================== Files 12 12 Lines 2226 2235 +9 ========================================== + Hits 1930 1939 +9 Misses 296 296 ``` | Flag | Coverage Δ | | |---|---|---| | unittests | `86.75% <100.00%> (+0.05%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Fredrik+Bagge+Carlson#carryforward-flags-in-the-pull-request-comment) to find out more. | [Impacted Files](https://app.codecov.io/gh/baggepinnen/ControlSystemIdentification.jl/pull/127?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Fredrik+Bagge+Carlson) | Coverage Δ | | |---|---|---| | [src/arx.jl](https://app.codecov.io/gh/baggepinnen/ControlSystemIdentification.jl/pull/127?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Fredrik+Bagge+Carlson#diff-c3JjL2FyeC5qbA==) | `89.70% <100.00%> (+0.25%)` | :arrow_up: |

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Do you have feedback about the report comment? Let us know in this issue.