This empty instance in then filled by the get() function: it loops through all
SIC variable dictionary and searches for all names which begin with the structure
name. For each valid variable it get()'s it as a SicVar attribute of the SicStructure instance. SicStructures support nested structures, as user or program can define any kind
of attributes (SicVar, SicStructure, or even standard Python objects).
For user convenience, the __str__() method (called by print myinstance or str(myinstance)) of the SicStructure class is redefined to print all (and only) SicVar and SicStructure attributes, with some details on them.