Fixed backward compatibility of cluster space (and expansion) files

Description

As a result of the improved handling of tolerance parameters (symprec, position_tolerance, ...; see !383 (merged)) their values are now expected to be stored in the cluster space (via cluster expansion). Since these values are not present in older ClusterExpansion files the code currently breaks when reading such files. This MR introduces a fix which simply sets these parameters to some default in order to be able to (re)initialize the ClusterSpace. In borderline cases where numerical tolerance has been an issue before this could lead to inconsistencies between ClusterSpace and the parameter vector stored in the ClusterExpansion.

Note that I did not add a unittest for this fix but rather excluded the respective line from the coverage since it is tedious to create ClusterExpansion file in an outdated format "live" during testing.