Add Universal Decimal Classification to Library Explorer

According to [Wikipedia](https://en.wikipedia.org/wiki/Universal_Decimal_Classification): "UDC is used in around 150,000 libraries in 130 countries and in many bibliographical services which require detailed content indexing. In a number of countries it is the main classification system for information exchange and is used in all types of libraries: public, school, academic and special libraries.[[16]](https://en.wikipedia.org/wiki/Universal_Decimal_Classification#cite_note-16)[[17]](https://en.wikipedia.org/wiki/Universal_Decimal_Classification#cite_note-UDC_Users_Worldwide-17)[[18]](https://en.wikipedia.org/wiki/Universal_Decimal_Classification#cite_note-UDC_Countries-18)

UDC is also used in [national bibliographies](https://en.wikipedia.org/wiki/National_bibliographies) of around 30 countries. Examples of large databases indexed by UDC include:[[19]](https://en.wikipedia.org/wiki/Universal_Decimal_Classification#cite_note-Large_collections-19)"

Currently, in Library Explorer you can choose either Dewey Decimal Classification or Library of Congress Classification. I think with such a big popularity UDC should be available in Explorer. It is not much smaller in usage than DDC which is used in 200,000 libraries in at least 135 countries