Warn about multiple values in telecom:medium

[francois.lacombe]

Dear maintainers,

It comes to my attention that mappers can add ;-lists as values for ​telecom:medium key on ​telecom=exchange or ​telecom=connection_point features.

Although it's not intended to cause any harm to the db, it's often not correct to do so, because exchanges or connection_point only have a single medium.
Using several values in medium key means you merge two independent feature in one. This because you see them in the same building but they should be independent.

As to encourage people to split such features, is it possible to raise a warning and stating they possibly merge several features please?