Missing values vs feature status

[mlindauer]

Reported by Chris Cameron:
some features have missing values
(either '?' or -512) for features within features groups that have 'ok'
feature runstatus (e.g., Titus/TTP-2016, Mira/MIP-2016) . I had always
thought this was something the ASlib format did not allow.

In both scenarios, we have only a single feature group.
If we strictly follow the ASlib format definition,
we would need to disable this single feature group if a single value is missing.
Thus, we would have no features at all for many instances.
However, the feature computation could be nevertheless successful even if we have missing values,
e.g., the reason for a missing value could be division by 0.

Retrospectively, I would say that it was a wrong design decision that
missing values imply a non-successful feature status.

As a hack to resolve this issue, we encoded missing values as -512
and not as "?".
Please note that there is no special semantic of -512 in ASlib.

[mlindauer]

The problem also exists in OPENML-WEKA-2017, CSP-Minizinc-Time-2016, and CSP-Minizinc-Obj-2016; however, the missing values are encoded as "?" in these scenarios.