There were problems when overriding the default behaviour of the rule…

…s. Moreover

they were moved to separate methods to resuce the complexity of the export method.

Various tests were added:
- Export tests that are based on a real workchain were added to ensure that a complex
  and real AiiDA graph based on workchains is exported correctly.
- Export tests based on a synthetic graph were added that explicitly check every
  flag that changes the default behaviour of the export set expansion rules.