Throw an error if `original` is used with any incompatible method

[gsong]

Pitch

@amotl: Skimming through the ops guide, section 4.5 reads:

As OPS provides access to data that is held inside the EPO, the OPS services (except
the number-service) require the input to be in one of the two EPO formats: epodoc or
docdb. The most common use case of the number-service is thus to transform numbers
that are given in domestic formats of different countries to the EPO formats that can
then be used for receiving published, family, register, legal or classification data.

This would tend to reinforce your suggestion to raise an early exception when using original in any method except the number() method. See also #81 (comment) for more info.

@gsong says:

We need to flesh out design details for this.

[mattkeanny]

In favour.
I guess that would qualify as "breaking change" though?