Error checking in sch.Component.addField

Created by: airwoodix

Hello,

these commits fix the error checking TODOs on sch.Component.addField:

• name and ref are made compulsory keyword-only arguments
• invalid additional fields are silently ignored

The function interface changed from a single dictionary to keyword based. This has at least two advantages: self-documentation is better and required arguments are explicit.

The ensure_quoted function is there because it is more natural to write comp.addField(name="foo", ref="bar") than comp.addField(name='"foo"', ref='"bar"').

The call to addField in the add_part_number.py script is modified accordingly.