IEV improvements: provide indication of an "absent designation"

[ronaldtse]

In the IEV ..... the string of 5 periods indicate that a term is explicitly "absent".

We should add the designation type "absent" to allow differentiate between:

• a concept without designations
• a concept with an explicitly absent designation

e.g. IEV 731-01-27 de and fr.

[opoudjis]

Add +absent boolean attribute to designation.

[ronaldtse]

Closed in b10cf99