Change type of Value_reference_pair.value

[s-heppner]

See #208

In the specification, the type was changed. List[Lang_string_short_name_type_IEC_61360] corresponds to ShortNameTypeIEC61360 in the spec. I'm personally lacking knowledge on DataSpec IEC 61360 to identify if this change makes sense, so I'll take it for granted.

See Part 3a, Chapter 6.5, page 37 https://nextcloud.s-heppner.com/index.php/s/JFHPPSW9D7KtgBC#page=37

[mristin]

The value type should be a string, not a short name type. For example, it can be an integer with 400 digits which does not fit the short name type, but is a valid IEC 61360 integer.

[s-heppner]

Decided in the WS AAS:

New data type: ValueTypeIEC61360 | string with max 2,000 and min 1 characters Introduce ValueTypeIEC61360 for DataSpecificaitonIec61360/value and ValueReferencePair/value

[mristin]

This change will be submitted in a separate pull request unified with #222.