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

[ekrich]

EXIP hits assertion failure at src/grammarGen/src/genUtils.c:268 when processing schemas with <sequence maxOccurs="unbounded"> containing multiple child elements.

$ ./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):

<xsd:complexType name="userDefinedType">
   <xsd:sequence maxOccurs="unbounded">
       <xsd:element name="COMMENT" type="xsd:string" minOccurs="0" maxOccurs="unbounded" />
       <xsd:element name="USER_DEFINED" type="ndm:userDefinedParameterType" minOccurs="0" maxOccurs="unbounded" />
    </xsd:sequence>
</xsd:complexType>

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