Cleanup range parsers

[roskakori]

The current parser for range.Range and range.DecimalRange are pretty messy and could use a clean up. Issues to be fixed are:

• Better code reuse; there's quiet a lot of copy and paste in there which might be collected in a common base class.
• The following cases should result in an improved error message:
  • 1 2
  • 1 x 2
  • 1 Tab 2
• Range.items should return a tuple instead of a list so it cannot be modified after Range.__init__()