add licence information to export XML

[kkrebs]

Related to #154 and #65.
atm we only show licence information on website, it is not part of xml.

[kkrebs]

It seems, that or are appropriate elements for licence information in MEI header:
https://music-encoding.org/guidelines/v4/elements/availability.html
https://music-encoding.org/guidelines/v4/elements/userestrict.html

[annplaksin]

So, that's a hard case for me currently... and I am very sorry for writing too much.
Those elements are the appropriate ones. They are used within <pubStmt> in the <fileDesc> in the head of a whole MEI file, to give licence information about the whole MEI file. (see example below)
The chucks of MEI we are using for the source catalogue on their own can't have this information on their own, because the publicartion statement within a manifestation describes the manifestation and not the metadata about the manifestation.

So, if we want to give the licence information for every chunk of MEI, I don't know how this can be done. Without having a <meiHead> with a <fileDesc>.
Is there a plan to provide a data export? In that case, the export could be done as a whole MEI header and contain the licence info.

The future xml versions of the editions, by the way, will have this licence info and retrieve the data from the catalogue for the header.

Example:

<mei xmlns="http://www.music-encoding.org/ns/mei" meiversion="4.0.1">
    <meiHead>
        <fileDesc>
            <titleStmt>
                <title>A small example</title>
                <respStmt>
                    <resp>Encoder</resp>
                    <persName xml:id="AP">Anna Plaksin</persName>
                </respStmt>
            </titleStmt>
            <pubStmt>
               <availability>
                   <useRestrict auth.uri="https://creativecommons.org/licenses/by/4.0/">Here is the place for the licence information for the whole MEI file</useRestrict>
               </availability>
            </pubStmt>
            <sourceDesc>
                <source>
                    <bibl>Example source</bibl>
                </source>
            </sourceDesc>
        </fileDesc>
        <workList>
            <work xml:id="exmplWork">
                <identifier auth="CMO">Example</identifier>
                <title>Drunken Sailor</title>
                <langUsage>
                    <language xml:id="en">Turkish</language>
                </langUsage>
                <classification>
                    <termList>
                        <term>Vocal music</term>
                    </termList>
                </classification>
                <expressionList>
                    <expression xml:id="exmplExpression">
                        <title>I am an example</title>
                        <relationList>
                            <relation rel="hasEmbodiment" target="#exmplManifestation" n="p.31"/>
                        </relationList>
                    </expression>
                </expressionList>
            </work>
        </workList>
        <manifestationList>
            <manifestation xml:id="exmplManifestation">
                <identifier>Some manifestation</identifier>
            </manifestation>
        </manifestationList>
    </meiHead>
    <music/>
</mei>