Added static method FormatCollection::getDisplayValue() (refs #4176)

Also:
- changed Report class to use FormatCollection::getDisplayValue()
- changed NormalAttribute::getTextualValue() to use
  FormatCollection::getDisplayValue()
- corrected PU tests
- removed private NormalAttribute::getTextualValue.+() methods
Merge pull request #959 from eguaj/3.2-A4176

Added static method FormatCollection::getDisplayValue() (closes #4176)