Support `<sequence maxOccurs="unbounded">` in schema grammar generation

EXIP hits assertion failure at `src/grammarGen/src/genUtils.c:268` when processing schemas with `` containing multiple child elements.

```sh
$ ./exipd.exe -xml -schema=ndmxml-4.0.0-master-4.0.xsd.exi,ndmxml-4.0.0-namespace-4.0.xsd.exi,ndmxml-4.0.0-common-4.0.xsd.exi,ndmxml-4.0.0-ndm-4.0.xsd.exi,ndmxml-4.0.0-omm-3.0.xsd.exi ndm-hdr.exi
```

**Found in CCSDS NDM/XML 4.0** (`ndmxml-4.0.0-common-4.0.xsd:1737`):
```xml

```

This is a valid W3C schema pattern `(COMMENT* USER_DEFINED*)*` but EXIP couldn't process it.

Start at the assertion in src/grammarGen/src/genUtils.c:268 and reproduce it with the provided ./exipd.exe command and CCSDS schema inputs. Trace grammar generation for the repeated sequence pattern (COMMENT* USER_DEFINED*)*. Done means the valid schema is processed without the assertion failure.

Written by the indexing model from the issue text.