add semantics tag name

[codinglist]

add semantics tag name

[coveralls]

coverage: 99.635% (-0.4%) from 100.0%
when pulling f36ec5c on codinglist:master
into c3ecbf3 on PHPOffice:master.

[Progi1984]

@codinglist Hi have you a MathML sample ?

[codinglist]

@codinglist Hi have you a MathML sample ?

codes:

use PhpOffice\Math\Element;
use PhpOffice\Math\Math;
use PhpOffice\Math\Writer\MathML;

include_once './vendor/autoload.php';
$math = new Math();

$semantics = new Element\Semantics();
$semantics->add(new Element\Identifier('y'));
$semantics->addAnnotation('application/x-tex', ' y ');

$math->add($semantics);

$writer = new MathML();
$writer->write($math);

error:

Fatal error: Uncaught PhpOffice\Math\Exception\NotImplementedException: PhpOffice\Math\Writer\MathML::getElementTagName : The element of the class PhpOffice\Math\Element\Semantics has no tag name in /xproject/vendor/phpoffice/math/src/Math/Writer/MathML.php:128 Stack trace: #0 / /xproject/vendor/phpoffice/math/src/Math/Writer/MathML.php(41): PhpOffice\Math\Writer\MathML->getElementTagName(Object(PhpOffice\Math\Element\Semantics)) #1 /xproject/vendor/phpoffice/math/src/Math/Writer/MathML.php(30): PhpOffice\Math\Writer\MathML->writeElementItem(Object(PhpOffice\Math\Element\Semantics)) #2 /xproject/test.php(16): PhpOffice\Math\Writer\MathML->write(Object(PhpOffice\Math\Math)) #3 {main} thrown in /xproject/src/Math/Writer/MathML.php on line 128

[Progi1984]

Hi. Thank you for your code.

I created a new PR #20 : I used your code and added you to the changelog as contributor.