Fixed handling of dimensionless quantities in UnitBase/to_system

Unit conversion had a bug in class UntBase / method to_system
for dimensionless quantities - fixed
Improved bug fix for dimensionless quantities

I improved my bug fix for dimensionless quantities in
class UnitBase / method to_system:
The fix is now done in the method 'score' where the
original error (division by zero) occured. The current fix
prevents the 'division by zero' happening at all while the
first version only prevented a problematic argument to be
passed in the calling function.
new tests for si/cgs conv. of quant. w/o dimension

I added two tests for the conversion of 'unitless' quantities
to the SI and CGS systems (Issue #1150).
Merge pull request #1214 from terben/unit_dimensionless_branch

Fixed handling of dimensionless quantities in UnitBase/to_system - Issue #1150