[MAINTENANCE] Format XML output of OAI-PMH

[stweil]

The current XML output is generated with a Fluid template. Therefore it is badly formatted with lots of empty lines and unnecessary whitespace.

[stweil]

Here are some numbers for http://HOST/oai?verb=ListIdentifiers&metadataPrefix=mets"|wc:

# minimized XML
       3      13    1766
# formatted XML (new code)
      49      59    2046
# unformatted XML (old code)
     172      59    6172

So this pull request reduces the size of the answer for this specific OAI-PMH request from 6172 bytes to 2046 bytes.
A minimized XML would reduce the size a little bit more and could be added as a configuration option in the future.

[sebastian-meyer]

Suggested change

	$dom = new \DOMDocument();
	$dom = new DOMDocument();

[stweil]

Note that the code already uses new \DOMDocument(); in function getMetsData. This is more safe and efficient than new DOMDocument(); which should only be used if this function were overridden in the current namespace.

[sebastian-meyer]

Codacy complained because you declared use DOMDocument; at the top of the file, but then added the namespace in this line anyways. The use-statement already makes clear that there is no such function/object in current namespace and the global version should be used.
The reason why getMetsData() uses new \DOMDocument(); is, that before your changes we didn't have an use-statement for DOMDocument. Strictly speaking we should remove the backslash from getMetsData as well, now.

[stweil]

Codacy complained about my new code before I added the use statement. I added it to fix the Codacy warning.

[sebastian-meyer]

Both is true: Codacy generally discourages using any namespaces in the code in favor of declaring all used namespaced functions and objects per use-statement.
So Codacy first complained about the missing use-statement and then complained about a namespace in code for which a use-statement exists.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 0.00%. Comparing base (411bfb8) to head (298c5cf).
Report is 1 commits behind head on main.

Additional details and impacted files

@@     Coverage Diff      @@
##   main   #1473   +/-   ##
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