Do not allow empty interfaces (iface) unless explicitly omitted

[ceremcem]

When a footprint class is inherited directly from Footprint class and "has" some other footprints and/or pads, it's likely to be forgotten to declare the @iface. Without @iface, it's not possible to detect unconnected pins.

Proposal

This type of errors should be prevented by requiring user to explicitly declare @iface to a magic value to indicate that it's explicitly left empty.

[ceremcem]

Note: Proper test case should be added.