Parser doesn't complain when degenerate component names are defined

[Laurie Nevay]

If you define an element in the parser, another element with the same name will not be used in the beamline - instead the first one will be used.

Example:

• mybend: sbend, l=1*m, angle=0.1;
• mybend: sbend, l=1*m, angle=1;
• l1: line(mybend);
• use, period=l1;

The resulting lattice has a bend of angle 0.1. No warning or complaint is produced.

[Jochem Snuverink]

fix issue #43 : add option to ElementList push_back to check for name uniqueness (false by default since used by beamline_list that can have duplicates)

→ <<cset e497969d58ba>>