choice generation bug

### 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.