!!Simulation execution

The following specifies the SimTree behavior in simulate and
collect runs:

A simulate run is required and therefore executed when

* for default batches (BM): The final results do not exist.

* for default batches (BM): The final results exist but
  batch results do not exist [Reason: Without batch results
  SimTree cannot determine whether the final results are
  created from a simulate run with the requested batch count]

* for default batches (BM): Default batch results exists but
  have not the same batch count as requested [Reason: Prevent 
  inconsistencies by differing number of default batches]. 

* for seeded batches (BM): Export results for the requested seed do not 
  exist or they exist but have not the same batch count 
  as would be created by the simulate run [Reason: Required result missing
  or invalid].

* for drops (IR): A drop export result with the same seed as requested
  does not exist [Reason: Required result missing].

A collect run is not required and therefore skipped when

* no export result files exists [Reason: Nothing to collect]

* final results exists [Reason: Whenever new export results are
  created, the final results are deleted (see consistency). When
  export results are cleared, final results are always also cleared.
  So it is always ensured that the final results are consistent with the 
  existing export results].

The user may define by properties

* whether batch results are generated for seeded batches. In most cases they are
  not required, so the default for this property is false.

* whether batch results are generated for default batches. To prevent
  simulate runs which are not required, the default for this property is true.

* whether drop results are generated. In most cases they are
  not required, so the default for this property is false.

* whether export results are deleted after collecting them in final
  results. This saves disc space but may require additional simulate runs.
  The default for this property is false.
