Class that can be used to check if a global identifier code is well-formed.
IMPORTANT: this doesn't check if the code is valid in the sense that
it doesn't check the code against a list of global identifier codes.
The code list provided with the ZUGFeRD standard only lists five codes.
There are more codes available (in ISO 6523).
We won't check the presence of a code in ISO 6523, but we'll check if
the code consists of four numbers.