Non-default simulation basename should have incremental postfix

The code that adds the incremental postfix to the basename only works if the default date-based basename is used. When the user sets the simulation basename, either from the CLI or the config file, results files are overwritten for posterior executions with the same basename. This makes it impossible to simulate several replicates with a non-default basename.

Postfix generation code should be moved from the object attribute definition to the object instantiation routine (->new()), so that any basename receives the postfix.