Use target from WithTarget in import format definitions

[dcolleoni]

This example shows how cashflow import format uses DataSource as targe

To fix this issue one should use the options available in the format:
ImportVariable.DefineFormat(Format, async (options, dataSet)=>
this options contain
public IDataSource TargetDataSource { get; init; }
to be used as target

As consequence:
improve the SequenceImportTest by removing the second call to Initialization.