choice generation bug

[Mike-M0]

Describe the bug

From my understanding: For any element which is marked as "choice" in xsd, the generated c structure should look like:
typedef struct mainType { union { typeA elementA; typeB elementB; ... }; int elementA_isUsed:1; int elementB_isUsed:1; ... };
currently all elements are added to the structure directly instead.

This is very inefficient in memory usage and can easily lead to en/decoding errors.

To Reproduce

This happens for all din, iso-2 and -20 types.
In multiple structures, e.g. in ParameterSetType -> ParameterType, where the parameter could be one of type per standard:
bool, byte, short, int, physical, or string, but never multiple at once.

Anything else?

The general functionality of union seems to be there.
The PGPData a union is generated, but in completely wrong way which overwrites itself.

[SiebrenW]

PGPData is not really used in a valid ISO15118 or DIN 70121 (see here: #56 (comment))

Though I think this is a case for excluding its generation altogether, because this is a recurring report, or generate a comment explaining the situation.

I think it's trivial to fix, but also trivial to bump something more important over in the process.