getConcordances includes XML:IDs of concordances, groups and connections or generates a stable ID for them

[DennisFriedl]

Description, Context and related Issue

Refs #209

Includes the xml:id of connection elements in the objects. This makes it possible to navigate to a certain connection via their ID, see Edirom/Edirom-Online-Frontend#268

Now returns something like

{
   "id" : "connection2",
    "plist" : "....",
    "name" : "2"
  }, {
    "id" : "connection3",
     "plist" : "....",
     "name" : "3"
}

How Has This Been Tested?

Tested with Edition Example and internal Korngold data.

Types of changes

• New feature (non-breaking change which adds functionality). The endpoint should work as normal as no existing values of the objects are modified.

[DennisFriedl]

I modfied this PR: getConcordances will now use the function generate-id to give each concordance, group and connection a unique ID if it does not have a xml:id. This Id is the same on every request and allows the navigation to e.g. a connection (see Edirom/Edirom-Online-Frontend#269) even if this connection was not explicitly made identifiable with an xml:id by the editor. However, present xml:ids are respected and will not be overwritten.

[peterstadler]

The code looks really good and I agree that this can be merged without breaking backwards compatibility.

Yet the test are failing (as expected) so you should update the test files (the expected results) within this PR.

[peterstadler]

Looks good, and tests are successful 👍