Abstract commonly used functionality from `EntitySource`

[sebastian-peter]

Especially with #926 it became apparent that some sources re-implement functionality that already exists in EntitySource.
This functionality includes (but might not be limited to) methods

public abstract void validate() throws ValidationException;
<C extends UniqueEntity> Try<Void, ValidationException> validate(Class<? extends C>, SourceValidator<C>)

, which could be reused by classes

• *WeatherSource (CsvWeatherSource might be a bit more difficult)
• *IdCoordinateSource
• *TimeSeriesSource

where * can be multiple of Sql, Csv, Couchbase, InfluxDb.

Probably the methods above have to be adapted to abstract the functionality properly.

In the end, we'll probably have AssetEntitySource, which takes over most of the current EntitySource implementation, and a new EntitySource which will be extended by AssetEntitySource and by the classes mentioned above.