Use units for internal calculations

All numeric values are converted to dimensional quantities just after
reading them in from MAD-X or other sources and converted back to float
values just before doing output.

This reduces the complexity of book keeping which value corresponds to
which unit by a great deal.
Do not display 'Vary', 'Knl', 'Ksl' in element popup view
Merge pull request #1 from coldfix/display-units

physical units