validation - configure missing value default

[GertjanBisschop]

When reading in data we should ensure we can specify what the expected missing value is.

• configuration <- @dirkdevriendt
• pass it to DataOpsAdapter

[dirkdevriendt]

This is currently meant to be configured as ObservablePropertyMetadataField (defining the property) and ObservablePropertyMetadataElement (setting the value on a ObservableProperty).

The challenge is that there is a variety of ways in which data can be defined as missing (with multiple values, meanings and contextual metadata, e.g. indicating the reason why the data is missing and/or what this means in terms of data quality or provenance annotations). Sadly no consensus could be found yet on what a sufficiently good internal gold standard would be for keeping that metadata. So we're in the exploratory phase where we implement is a simply as possible and as flexibly as necessary. (meaning for this question just one "special value" that should be replaced with None?)

This can then be handled in the first stage of the data ingestion process.