Error checking in sch.Component.addField

[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.

[poeschlr]

Sorry for the delay, this looks good to me. (To be honest the sch class is not something we take great care of anyway as we do not really need it for our task of maintaining the library.)